That all depends on how much real money you want to spend as there are several options. Currently these are:
Starter Kit
450 Robux - Buy for $4.95
1,000Robux - Buy for $9.95
2,750 Robux - Buy for $24.95
Super Value
6,000 Robux - Buy for $49.95
15,000 Robux - Buy for $99.95
35,000 Robux - Buy for $199.95