Rehearsal studios are indeed a luxury. Different people have different circumstances, but the kind of solutions include:
- Homes / sheds / garages
- Back rooms/cellars/attics of shops/pubs/etc.
- School/church/community halls
- Offices after hours
- The street
... through a variety of legitimate and illegitimate means -- I'm sure there will be bands out there who trespassed in some derelict building for all their early rehearsing.
Consider that a full band including a drumkit can fit into a small caravan, if they're willing to squeeze up.
If you're an amplified band, consider that you don't have to rehearse at full volume every time - especially if the drummer has some electronic drums (if he doesn't, then rehearse wherever he practices drums!)
Consider that amps and other electronics can run off batteries; you don't have to rehearse somewhere with power.
If you really want a studio, you could ask if they have lower rates late at night or early in the morning.