Hmm, it really shouldn't cost you 50 mil. Before I see some people putting it for 42~ish. There's one on sale for 55 mil and it's +7... makes no sense.
Anyway, I guess for now go for the cheaper weapons. Like what axle said too. But the only cheap +crit weapon you'll find is soldier skeleton. They shouldn't be that hard to fight I think. I dunno your level though. >.<
At the moment of posting the lowest prices are:
2x doppel card - 44mil (total)
1x mobster card - 1.5 mil
1x alligator card - 3.5 mil
1x turtle general card - N.A. 27 ads if you want it now
+10 s. jur - 700k
That would add up to
49.7 mil for +10 DDMA
So in total zeny price that would be around 67mil for +10 DDTA
taken that T = 22 mil also
Even if your calculations are right, cards lose value when slotted. No weapon like that costs 67 mil.

No one will buy it. Even a DDTP cost 63~65 mil.
And if anyone's selling you doppel for 22 mil... don't buy it. =\ Excuse me, nvm... it seems prices shot up for some reason. It'll eventually go down again. ><;