How many of you are visiting. Which area of London do you want to stay in (what do you want to see, do?) Greater London is 609 square miles large so you don't want to be in the wrong bit!
Is it 50 euros per room a night or per person per night? Do you need a nice hotel or will any bed do?
Once you know these things you're ready to search.
My favourte hotel search places are:
www.opodo.co.uk, www.expedia.co.uk, www.laterooms.com, www.hotels.com, www.lastminute.co.uk, www.smoothhound.co.uk
Once you've found one you like the look of for the price you like, make sure you check the reviews on www.tripadvisor.com to make sure you're not booking a flea pit.
There are some nice reasonably priced hotels in London but there are also an awful lot of really horrible ones too so make sure you do check before you book it.