It is pretty much pay on put. You charge up your space account with safecoin and will be told how much space is left based on current prices (so if you leave it and store nothing your space will likely decrease). So each store will incur a cost at that time on the network. The network knows the cost of all puts in real time. I assume space will decrease in cost very fast and there will be an opportunity for your client to pay frequently rather than in larger chunks, thereby keeping your cost per Mb as low as possible.
The network aims for 20% over supply and will flatten out farming over that amount, as farmers reduce or space required increases then the network increases farming rate to get back to 20% above average.