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
http://www.dealsghost.com/hotel_discounts_offers.html
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.