This seems like a no brain-er, but if I were to buy *anything* from the Marketplace, it would be more storage for my personally made textures. Right now (for free, mind you) we get only 100MB. In the last week I have uploaded 13MB. In one week alone I have uploaded more than a tenth of my quota. Being able to buy more storage quota is a must.
But it also must be price reasonably! Right now the price for a GB of hdd space is at about $0.06 a GB. Now I am not saying that we should only have to pay six cents for each gigabyte we want, just that this needs to be taken into consideration when creating the price(s). $5-$10 (one time fee) seems reasonable for a gig of net storage, $10 being the most I would pay.