Not the Hyatt, not Caesar's Palace, just a decent hotel with a clean shower and clean linens. A place that's not roach-infested, but not valet parking either.
Depends on when you are going and which area you will be staying in....But on an average cost for a day will be around 2000 Rs i.e 45 USD
If you book online you can get many offers too like check this link for few of them
Go to Tunga International or Tunga Paradise or Kohinoor Exec or something similar...they all range from 4-5K INR and are very very nice hotels...
Maybe you can check Mumbai's official website for the list of hotels with their corresponding rates.
Check TravelGuru : http://www.travelguru.com/
You can narrow your search according to your budget, and see what reviews other travellers have made about each hotel. That will give you a better idea of what to expect for the price.