I'm really lucky, I grew up in Brighton and parents still living there so I just go and stay with them for Pride
Maybe I should advise them to open up as a B&B for the weekend, they'd obviously make a killing if everyone is having problems with accomodation...!
Staying just out of town and commuting in is a good plan, but be aware that taxis will be few and far between (as they are on any regular saturday night, let alone pride!), and if you're planning on going out in the evening you'll have to leave pretty early to get the last train... Thus, night buses:
N7 HOVE - Hove Station - Seven Dials - Brighton Station - Queens Road - Old Steine - St Georges Road - County Hospital - BRIGHTON MARINA 30 minutes all night
N25 PORTSLADE STATION - Palmeira Square - Churchill Square - Old Steine - St Peters Church - Lewes Rd - SUSSEX UNIVERSITY 30 minutes all night
N69 BRIGHTON - Hurstpierpoint - Hassocks - Burgess Hill - HAYWARDS HEATH Nightclub Bus
N98 BRIGHTON MARINA - County Hospital - Brighton Station - Town Centre - Hove - Portslade Stn - Hangleton - Downs Park - MILE OAK Nightclub Bus
N99 BRIGHTON STN - Town Centre - Whitehawk - Woodingdean - Rottingdean - PEACEHAVEN
Not sure whether sussex uni rent out their rooms in the summer - maybe an option to check out? Unis usually have nice single rooms, often with ensuites at a reasonable price during student holidays.
NB Don't stay in whitehawk or woodingdean...! (I doubt there are hotels there, but just in case!)