I’m in the process of collecting all of Ancient Mew variants too. This video was helpful to me starting off: - YouTube.
In terms of prices, if you’re talking about PSA 10s, Ancient Mew 2000 goes for about $150, Ancient Mew 2019 goes for about $100, Ancient Mew II goes for about $150, and Ancient Mew I Error goes for about $500. There isn’t a whole lot of recent data on Ancient Mew I corrected. It’s rare enough to where the seller can dictate the market, and during a time like this where all card prices are trending upwards, this is the variant that will be the most difficult to obtain, both availability and money-wise. There’s some good discussion about it on this thread: www.elitefourum.com/t/ancient-mew-collection/27297/1.
Overall, the cost to complete this collection with PSA 10s is significantly more expensive to complete now than it was just a few months ago. Whether that trend will continue, or for how long, is anyone’s guess. If you’re looking to complete this collection for a cheaper amount, I would recommend looking for lower graded cards. Even a drop from a 10 to a 9 can make all the difference price wise.
Good luck.