My suggestions are more like interesting groups of cards rather than full stand-alone sets, as most of these cards are part of much larger groups of promos. Additionally, most of my knowledge of promo cards is from Japanese sets, but if you’re willing to branch out a bit, I’d consider the 8 Japanese Game Boy Promo cards + the English Black Star Promo GB stamp Meowth to make a clean 9 card set for a binder page. More information on these awesome cards is available from Mr. Bubbles’ post here: Promo Showcase – Pokemon Card GB Promo .
There are also the Pikachu Records CD Promo cards as Joer has recommended. You could additionally go for the Natta Wake set, of which there is a wonderful article by pfm here: "How I became a Pokémon card" (Natta Wake) - A guide to the manga and cards
One thing I don’t see mentioned is the set of 4 cards from the CoroCoro Illustration contests from 1998, there is the Poliwrath, Snorlax, Mr. Mime, and Mewtwo. These cards shouldn’t break the bank either (though I wouldn’t consider them cheap).
There is the 11 cards from the 10th Movie Commemoration Set and the 2 Alto Mare E-Series Movie Promo cards
For English, maybe grab the Stamped NY Pokemon Center Ho-Oh and Rapidash promos? You can also collect all of the “For Position Only” E-reader cards, but good luck trying to do that XD
Hopefully at least one of my suggestions was interesting!