You can get started for around $500 with a Speed Graphic, older (heavy) monorail, or one of the more basic woodfields, and a normal lens. You can process film in trays, which you probably already have if you do your own darkroom work. You'll need a light meter (but you can use the meter in your 35mm camera for now), a darkcloth, and a tripod (a Tiltall or something of similar character will support most 4x5" field cameras, maybe something a little heavier for a bigger monorail). If you make contact prints for now, all you need is a clean sheet of glass and a slab of foam rubber. Later on, if you need an enlarger, you can find something like an Omega D2 for a few hundred dollars. Darkroom equipment is cheap these days.