Mara's is 5/75 rarity (Qlvl 80).
So the odds of rolling a Mara's on any given gamble (appropriate char level of course!) are (1/2000)*(1/15) = 1/30,000.
If you want to have that 64% chance of rolling a Mara's, you'd have to buy around 30,000 amulets. If each amulet cost 45.5K (best gheed's and Edge RW), you'd need $1.365 B. That's 1,365,000,000 gold (about 15% less than that if you're reselling them all).
If you were going the longshot route and wanted only a 10% chance of rolling a Mara's, that would still cost you:
x=number of rolls
(1-((1-((1/2000)*(1/15)))^x))=0.1
(1-((0.999966666...)^x))=0.1
(0.999966666...)^x=0.9
log(b=0.999966666...)0.9=x
x=3161
3161 gambles for you to have even a 10% chance of getting Mara's. That's $143M for a 1/10.
The odds are better than getting a Zod. But you may find that Zod while collecting enough money to gamble for the Mara's (= p)