Pay to play is something that is often talked about on the internet and a practice this is employed by some gig promoters.
If you are a band and you are asked to pay a fee to play a gig then you need to seriously think is this gig worth doing. Most of the time the answer is no. Only on extremely rare (and I mean extremely rare occasions) should you consider to pay to play a show.
The pay to play concept should not be confused with promoters asking you as a band to help sell tickets to your fans. Some smaller gigs and promoters will ask bands to help sell tickets to their fans and receive a commission on sales instead of receiving payment for a show. This is still employed by promoters putting on lesser known original acts.
This is not a bad thing. To be honest if you are playing your own material and getting out on the scene and doing some support slots this practice is the most common you will probably encounter. You should only demand a fee if you have invested in building a significant following or invested in noteworthy marketing.
However you should not be expected to sell a specific number of tickets.
Small promoters are taking a gamble by putting on unknown acts. Which is fine its their choice. However if you expect a fee when you have not invested in developing your own following then your unlikely to be attractive to promoters.
The other option is to simply put on the event yourself. Hire the venue and promote your own performance. Then you are in control. Most small venues allow you to hire and put on your own night.
Promoters will do their best in promoting your gig. But as mentioned they do gamble on unknown acts and commonly do not offer a fee for every band that plays. Typically only a commission.
Best advice is to put on your own shows and definitely invest in marketing your band. Treat it like a business and forget the cost of rehearsals as part of your investment. Your rehearsals help you develop your product but do not offer anything in appeal to promoters of other clients.
This is a sensitive subject amongst bands.