FYI: At the moment the purchase amount is to spend one SAFEcoin for a "PUT" balance.
The balance is decremented every time a "PUT" (send message say) by an amount determined by a factor of the "Farming Rate"
For instance today a "PUT" might reduce your balance by 0.0000000001 of a SAFEcoin and tomorrow it might reduce your balance by more or less. In the RFC the balance is stored in a 64 bit integer and reduced by a factor of the Farming Rate.
In effect you could get as low as 1 "PUT" per SAFEcoin (Extremely unlikely) to as high as around 1,000,000,000,000,000,000 PUTs per SAFEcoin. The amount each PUT reduces your "PUT" balance depends on the current state of the SAFEnetworks resource allocations which is expressed as a figure called as Farming Rate.