Estimate hostel nightly costs.
Hostel pricing is a low nightly rate with a cluster of small additions, and the additions hurt most on short stays. A linen fee and a membership charge are one-off, so spreading them across two nights raises the effective rate by a third while spreading them across ten barely registers — which is why the effective nightly figure, not the advertised bed rate, is what you should compare between properties. Locker fees are the exception in that they recur nightly and scale with the stay.
Hostel stay cost
Total = bed rate x nights + linen fee + locker fee x nights + membership; effective rate = total / nights
Because one-off fees are spread over few nights. On a two-night stay a linen fee and a membership charge can add a third to the advertised rate; on a ten-night stay the same fees add a few percent.
Only if you will use affiliated hostels repeatedly. A joining fee that pays for itself across a multi-stop trip is poor value for one stay, and many hostels charge a small per-night supplement to non-members instead.
Yes — a bed in a large dorm typically costs 30% to 50% less than in a four-bed room. That gap is usually larger than every extra fee combined, so it is the first lever to consider.