often they're free on certain nights.
Have put on a lot of gigs at uni, venue cost on the whole goes from free for the basement of a pub/pub function room on a weekday night to over £100 for a largeish small gig venue. Main costs on top of this are PA hire (£20 from a mate of mine, would be a lot more otherwise) and sound guys (depends on the size of the gig, if its a small gig I tend to do it, bigger all day gig etc and we hire someone at a cost of £50.
Its a fair bit of hard work in negociating with bands/promoters etc to get them to play, also watch out for riders- they'll often eat into your budget quite a lot and in many cases the provision of one will be part of the terms of the contract.