Support slots are sold as exposure. Before you say yes, build the budget. Here is a worked example for a four-week run with a two-person crew.
Opening for a bigger act can be the best thing that happens to a developing artist. It can also be a four-week loss that takes a year to recover from. The difference is whether you ran the numbers first.
Support fees for club and theatre runs commonly land in the low hundreds per night. Merch is the real upside, but only if the headliner allows a table and does not take a large cut. Ask about both before you accept.
Four weeks, 22 shows, a van, two people plus the artist. A conservative estimate puts costs around $14,000 and support fees around $6,600. Merch at a modest $180 a night adds roughly $4,000. That is a loss of a few thousand dollars, before you count the exposure.
Sometimes yes. If the headliner's audience is your audience and you can capture it, the run pays back later. If the crowd is wrong or you cannot sell merch, it is an expensive holiday. Do the budget, then decide.