This achievement can be glitchy right now. Here's some potential fixes which people have found:
Claim ownership of the creature which will have all of the upgrades purchased.
Have your co-op partner (the one not getting achievements) unlock the final upgrade.
The amount to upgrade varies by character. The final ability can only be obtained by finding the Soul Gems in the different chapters. The locations of these soul gems are as follows, as they can be found pretty much in the open in these levels:
- Spyro - Crawling Catacombs
- Gill Grunt - Treetop Terrace
- Trigger Happy - Cadaverous Crypt
The easiest way to get money would be to replay the final couple of levels using the chapter select after finishing the main story if you have not collected enough money on characters during the normal progression. It is important to note that any money beyond full upgrades is essentially wasted.