How much are the phone rentals?

Last time I got a sim card with ntt docomo but was a bit disappointed with their speed and signal (frequent dropouts between Tokyo and Matsumoto, Tokyo and Sendai). I'd also like to know the price in case something were to happen to my own phone. I'd be looking at renting something like a samsung galaxy s5 for three weeks. No voice needed, perhaps just 2gb data. Also, how much is the deposit for rentals?

If you think renting a phone would give you the same service as with a regular Japanese contract, that's probably not true.

Besides, Docomo is the best network in coverage. If you didn't have signal with them, you woudn't have had any on any other network either.