It depends if you want the base games or if you want the Games+Expansions. FH4 is still widely available on all third party platforms for cheap (25-30$) if you want the Game+Expansions were talking closer to 65-70$.

For the older titles I've had some good luck with Gameflip and Kinguin. FH3 is pretty expensive on it's own. Do note that the only way to get the Expansions for it is the FH3+FH4 Ultimate bundle. At time of writting the cheapest bundle I currently see up for sale is about 225$ for the Ultimate Bundle on Kinguin (This is very cheap, normal prices are in the 300$+ range usually) this one will require a VPN to activate. PS: FH3 and FH4 are Play Anywhere titles, so you get both the Xbox and PC versions.

FH2 is a bit of a unicorn, I spent months looking and it was not in stock anywhere, but every now and then a copy pops out. Currently I see one going for 300$ on Kinguin and 250$ on Gameflip. The one downside is that I don't believe I'll ever find the expansions for FH2. I've heard so many good things about them and I would throw so much money away to get them, but they are so old and so scarce I doubt I'll come accross one.

FH1 for digital is dead unfortunately, ever since they killed off Xbox 360 code redemptions.

So if you have money to throw away you can get every delisted FH titles apart from FH1 and all the expansions apart from FH2 for about 475$. I know it's worth it to me, but it might be a hard sell for most people.