Sleeping in the Black Forest is not especially cheap, but moving around it is effectively free — nearly every guesthouse hands you a KONUS card covering buses and trains — so the budget comes down to where you sleep and how often you pay to go up a mountain.
The KONUS guest card
Handed to you when you register at any of roughly 150 participating Black Forest accommodations, it gives free travel on buses and regional trains for the whole of your stay, across an area running from Pforzheim to Basel and from Karlsruhe to Waldshut. Confirm your host takes part before booking — it is by far the biggest single saving here.
Walk the Westweg
About 285 km along the main ridge from Pforzheim to Basel, waymarked with a red diamond on white and laid out in 1900 by the Schwarzwaldverein, whose volunteer wardens still maintain it. Roughly 9,000 m of cumulative ascent over 11 to 14 stages, splitting into a western and an eastern variant near Titisee; stages end in towns with beds, so no tent is needed.
DJH hostels in Baden-Württemberg
The regional branch of the German youth hostel association runs houses across the Black Forest and is part of Hostelling International, so an HI card works. It also sells combined rail-and-bed packages at a fixed price, which is the cheapest reliable way to sleep outside the towns.
Wutachschlucht
The gorge in the south-eastern corner is free to walk, and the classic route runs from Bachheim down to the Schattenmühle past steep rock sections. A dedicated Wanderbus Wutachschlucht links the trailheads in season, so you can do it one-way without a car or a second driver.
What a day costs
Prices you can check today: the Triberg waterfalls are €9 in summer and €7 in winter, €8.50 and €6.50 with a guest card, with a €20 summer family card and under-6s free. Caracalla Spa in Baden-Baden is €21 for two hours or €35 all day. Europa-Park starts at €56.50. Buses and regional trains are free with a KONUS card, so on a pure hiking day your only real spend is lunch.