1 Account Upgrade = 8 AU
8 AU = ~8 Million MP

For Account Upgrades other than the treasure map ones, there is a 1 in 3 chance you will get the enchanted plushie.

FIRST TRY

LE Plushie = 7-8M
Costume = 1M
Other items = 1M

8M spent, 9-10M earned for a profit of 2M at the very most.

SECOND TRY

LE Plushie = 7-8M
Costume (x2) = 2M
Other items = 1M

16M spent, 10-11M earned for a loss of 5-6M

THIRD TRY

LE Plushie = 7-8M
Costume (x3) = 3M
Other items = 1M

24M spent, 11-12M earned for a loss of 12-13M

CONCLUSION

You could argue that you're buying it to save the items until they retire and inflate, but even then you're saving more of that hard earned MP by either buying what you want in trades/auctions or setting up a wanted ad and getting what you want dirt cheap.

Those are just my thoughts, let me know what you think in the comments!