RESIDENT EVIL 5

All 8 Versus characters cost 2,000 EP (Exchange Points) each in the Xbox One version (compared to the original 360 release, where costs ranged from 5,000 to 50,000 EP). In total, you need 16,000 EP to unlock them all. You earn EP by completing levels in campaign, matches in Versus, and completion of stages in Mercs United. For example, completing a campaign level with an S rank will give you...
- 1000 EP on Amateur
- 1500 EP on Normal
- 2000 EP on Veteran
- 2500 EP on Professional

The process is a lot less grindy here than it is on 360 thanks to balanced and reduced costs per character, so Exchange Points can stack up pretty quickly.

If you are not bothered about actually using these characters and prefer to save points for unlimited ammo on weapons, save the ep points until you can afford all of them in one go.

Buy them all together but then press the guide button and quit game.
You will get the achievement and when you reload you will have your ep points back.

This is a complementary guide to both of the above, since I just had to figure out on my own where to actually unlock the characters lol.

This game has a very badly designed interface (I love Biohazard/RE series) so in order to actually unlock the Versus characters you have to go to BONUS FEATURES and then scroll aaaaaaaal the way down to the bottom where you will be able to buy Versus characcters A through H. You need to hold the left stick down for a couple of seconds as there are a lot of stuff in there.

Bulletpoints from the previous excellent guides:
- There are 8 unlockable Versus characters and each one costs 2000 Exchange Points
- If you have the 16k EP to do unlock all of the characters in one go and then hard-close the game (Guide button and then quit, I also erased the local save game to be sure :P)

For anybody who has bought all characters but the achievement hasn't unlocked, here's what I did to get it without losing everything:

1) Go to Manage Games and Add-ons and delete your save data ON CONSOLE ONLY - leave the Cloud data

2) Go to Setting, Network Settings and select Go Offline

3) Start up RE5, create your new save data and... well, grind up the 16k points again. I found Mercenaries to be the best way to do this - I used Rebecca personally but I think any character should do. You can easily get an A-rank (40k points) for 1,500 EP, which means 11 games in all. If you can get S-ranks (60k points), you'll get 3,000 EP per game, cutting the number of games you need in half. Whatever you choose to do, stay Offline the whole time.

4) Go back to Bonus Features and, before doing anything, go to Network Settings and select Go Online (without closing the game). Unlock each character and then DO NOT leave the menu. Wait about a minute to see if it unlocks. If it doesn't, press the Home button, close the game, and Go Offline before opening the game to try again.

6) If the achievement finally pops, close the game, restart your console and open the game. You'll be asked to select which save data you want to use, Local or Cloud. Select Cloud and you can recover your original save.

Hope this helps anyone who was afraid they'd have to start entirely from scratch.