As already stated these figures can vary significantly and there are lots of variables, but the CAA airline profit/loss accounts might be quite a useful read:
Major UK Airlines Profit & Loss Accounts 2007
For example, the data shows that Monarch Airlines spent £37,779,000 on aircraft leasing during that year. They have about 30 aircraft so that's over £100,000 per aircraft per month. They mainly operate A320/A321/B757 sized aircraft.
Virgin Atlantic spent £208,870,000 leasing their 35 or so much bigger (and more expensive) B747/A340 aircraft - closer to £500,000 per aircraft per month.
Not a perfect calculation by any means (both airlines might actually own some of their aircraft outright for all I know and so they won't pay leases on those), but it is decent for ballpark figures.
When you look at the figures for an airline like Virgin it becomes quickly apparent that aircraft leases at £208m/yr are far from the biggest financial burden - £550m goes on fuel, £218m on pilot and cabin crew wages training and other expenses, £173m on maintenance. Total annual expenditure in fact is £1,994m
Not sure why anyone gets into this business really!!