Costs & Money
Given its capital city status, Madrid is not overly expensive. The Euro exchange rate will, of course, determine the price for many visitors, but if you shop around for accommodation, food and choose the right times to visit the sights costs can drop significantly.
Budget
At the budget end (staying in hostels or hostales, buying your own food, walking and visiting mostly free sights), you might be able to get away with about €50/person/day.
Mid-Range
A more typical budget for most visitors (staying at standard hotels or hostales, eating out once a day, going to a range of sights and taking public transport), would be around €80/person/day.
Luxury
It would take more like €150/person/day to get a high-end experience, with upmarket hotels, meals at the full range of restaurants and visiting all the sights.
Note that traveling solo will probably raise any of these per-person-costs by at least a third for all except budget travellers.