It's also worth mentioning that Hasselblad cameras - in fact, anything with 'Hasselblad' written on the front - are, like Leicas, exorbitantly expensive. Lenses alone are worth more than most decent-quality 35mm SLR
The aforementioned 501CM, for example, sells for £2,500 with a case, a lens, a film magazine and a strap - the lens alone sells for £1,300, whilst the strap and case together cost £70. For a strap... and a case. Meanwhile, a '300mm Tele Super Achromat FE f2.8 supplied with Apo-Mutar 1.7XE Shade and Polarizing Filter' goes for a cool £13,000.
Therefore, there's a thriving clone market - the Ukranian Kiev 88 being the most famous imitation, although the Mamiya RB 67 runs it a close second. These aren't particularly cheap, however - but they're cheaper than a Hasselblad.
If you want to experiment with medium-format photography the absolute entry-level is probably the ultra-tacky Chinese Holga 120S, a plastic box with a plastic lens on the front and minimal controls.