Playing before a well-known act looks like a promotion and frequently is not, at least financially. What a support slot returns depends on what the smaller act can extract around the fee.
The fee rarely covers the costs
Support payments are modest because the headliner is selling the tickets, and the promoter's budget is built around the name at the top of the poster.
Against that sit transport, accommodation, crew wages and equipment, all of which scale with the length of a tour rather than with the size of the fee.
On larger tours a support act may pay for the position outright, treating the slot as marketing expenditure rather than as paid work.
Audience access is the actual product
A support act performs to a room that assembled for someone else, which is a form of reach that no amount of advertising reliably buys.
The value depends on whether that audience overlaps with the support act's potential listeners, so a poorly matched pairing produces attention without conversion.
Acts and agents therefore weigh the fit closely, because a large room full of the wrong people is worth less than a small room of the right ones.
Conditions on stage are constrained
The support act typically receives a reduced set length, limited stage space and whatever sound and lighting remain available around the headliner's setup.
Soundchecks are short or absent, so the band must be able to deliver without the adjustments a headline show would allow them.
Doors open partway through in many venues, meaning the first songs play to a room still filling, which shapes how a set is ordered.
Merchandise decides the economics
Sales at the venue are frequently the difference between a loss and a break-even tour, since attendance creates the moment of intent.
Venues take a commission on those sales, and the rate is negotiated in advance, which is why merchandise terms feature so prominently in tour discussions.
An act with a strong table can survive a poor fee, and one without it can lose money in front of a large and enthusiastic crowd.
The relationships outlast the tour
Support slots are distributed through agents, promoters and the headliner's own preferences, and performing well makes the next opportunity considerably easier to secure.
Crews and tour managers move between acts constantly, so a reputation for professionalism circulates faster within the industry than any review does.
That accumulated standing, rather than any single tour's balance sheet, is what makes the position worth taking on unfavourable terms.