I think the exciting feeling fading over time a bit is normal. Just before you purchase the card, and even more so just before it arrives, the anticipation of it is at its peak. Especially for a card you’ve wanted for a long time (and even more so if that card is the final piece of a goal you’ve been working on for multiple years imo).
Having said that, I defintely don’t regret buying the cards that fit that description. A total of three purchased cards come to mind in the eleven years I’ve been collecting (although just the last two were also expensive in addition to rare grails):
Japanese unlimited edition WCP Seviper
I’m going over the three in chronological order, but this one wasn’t too expensive. Iirc I paid like 50 bucks for it. Nonetheless, it was a grail, and marked the last card to 100% my all-language TCG collection of my favorite Pokémon, which I had been trying to complete for 3.5 years at that point.
The feeling of finally 100% completing that collection when it arrived was almost euphoric, and yes, now that it is complete and I’ve been focusing on other collection goals, the feeling is a lot less than it used to be. Nonetheless, I’m still very happy to have this complete collection of my favorite Pokémon, and can still append to it with new releases over time, and that feeling won’t ever go away.
English Art Academy Pikachu by Ashley
This one hits your category a lot more, and is also a lot more special to me. Collecting all Pikachu in all languages has been my goal when I came back collecting in mid-2015, so I’ve been working on it for eleven years already. And although I’ve adjusted this collection goal to just the first 25 years for all languages (aka, up to February 27th, 2021 - the 25th Pokémon Day), since it became too expensive to continue at the same pace, I still continued with every single English Pikachu variation after that date (plus some exceptions: one of each non-English unique artwork, and all Base Set reprints).
When I was contacted by chibi/Sam, who recently upgraded his BGS-8 copy to a BGS-9, asking if I wanted to buy this card - the final English Pikachu TCG card I was missing, and just one of ten missing Pikachu cards to ‘complete’ my Pikachu collection - I was at first a bit scared of the price (for context, this was in early Fall 2024, and we settled on a price of 14k USD for it), but I simply couldn’t let the opportunity pass.
I knew that if I didn’t buy it then, I likely wouldn’t be able to afford it anymore in the future and possibly never own it, and 100% completing the English portion of my Pikachu collection was too great to pass on. I may have already had most other languages at 100% (for the first 25 years), but English and Japanese both feel that much more special, and even more so since I had continued with collecting all English Pikachu TCG cards even after the 25th anniversary. So even though it costed more than the car I had bought half a year prior and it still to this day is my most expensive bought product ever, minus my house, I’m still very happy I bought it and don’t regret it one bit.
German Surf-Pikachu promo
This one is the most recent, bought in September last year. Right now, every one of the missing Pikachu cards are grails for me. Five of them I’m simply priced out of (4x Japanese Art Academy cards, which are currently going for 150k-250k each, and the Pikachu SNAP, which is valued at roughly 1M-2M - if it would ever re-sell in the first place); and the other three (German Flug-Pikachu, Spanish Pikachu Surfista, and Spanish Pikachu Volador) I’ve only seen twice each in the 10+ years I’ve been collecting. This German Surf-Pikachu is in the same category as those three, in that this was the third copy I had seen, and only the first I had the opportunity for to buy.
So obviously, I jumped on it with everything I had. It ended higher than I was expecting tbh. But, lucky not too high, since I basically paid my absolute max for it by emptying my - at the time already low - savings account, and I even paid groceries and gas on credit after the purchase that month, while my regular bank account would handle the automatic payments (mortgage, insurances, internet/electricity, multiple monthly charities, etc.).
I’ve also bought some cards in December, but I’m honestly still saving money and not buying anything rn due to those two purchases (plus additional non-collection purchase in the form of a solar battery, which will be installed later this month).
Anyway, considering how rare this card is, and how this was the first and only opportunity to buy it in 10+ years time, I defintely don’t regret buying it, that’s for sure. It may be a bit annoying how behind I am with new releases today (over 150 cards already across my different collection goals, and counting.. 
), but I’m not too worried about their availability and will add all of them eventually (hopefully all later this year).
Anyway, sorry for the wall of text, just wanted to share my own experiences with it.
Whether it will be worth it for you, is completely up to you. 
None of us can make that decision for you, since it depends on so many different factors, like your financial situation rn or after the purchase; what the availability of the card in general is, even ignoring its price-tag for a moment; what makes the card so special for you; how would you feel if you do decide to pass on it (both rn, and down the line); etc.
My motto with collecting has always been: “You’d only have to spend the money once, and then you can enjoy the collectable item for the rest of your life.” (Although I’m also the first to admit it’s a pretty bad motto to live by, ngl. 
)
Greetz,
Quuador
