You ever wonder why one-person plays got so popular? Sure, they're the ultimate vehicle for an actor, and, in fact, several of the best plays I've ever seen - for example, Anna Deavere Smith's Fires in the Mirror - have been for one performer. But it sure is cheaper to pay one performer than ten, isn't it...
It's a sad fact of theatrical life these days that it's often not enough for a play simply to be good. It needs to be affordable. If you read through the latest edition of the Dramatists Sourcebook, so many companies have guidelines that include a "small cast" or "cast limit of" - followed typically by a relatively small number. The number might be as low as four or as high as twelve or more, but the bottom line is that reducing the number of people the theater has to pay to produce your play is always going to make it more attractive.
Have a character that only comes on for a line? Ask yourself if there's another way to fulfill his function. If not, can one actor play multiple roles? I always recommend writing in any suggested doubling yourself for two reasons. First, if there's a thematic reason you want a particular double casting, don't leave it to chance. For example, in my play Ben, the actor who plays Baxter, a gay restaurant owner who befriends a homeless teenager, also plays Dryer, the social worker who had molested the teen. I made that specific doubling choice both to underscore the teen's distrust of Baxter's good intentions and to highlight the stark contrast between Baxter and Dryer. The best doublings (or triplings, quadruplings, etc.) not only save a buck, but they serve the play as well.
Of course, keeping the cast size down isn't the only way to make your play affordable. There are also the dreaded "technical requirements." The most important is the set. Plays that can work with a single set or make use of suggested settings (i.e. a few set pieces, rather than a full build) are infinitely more likely to be produced than plays that require multiple, elaborate settings. Not only do the latter cost more to build, but the more involved the set changes during the show, the larger the crew that needs to be hired.
While the set is the most expensive technical element, it's not the only one. Shows that require elaborate costumes (especially period costumes), special effects or lots of perishable or unusual props are also more costly, either because of the materials, or the crew or equipment rental needed to make them work.
This is not to say that you should compromise your vision simply to save a buck. But often, the need to save a buck forces you to come to a better understanding of that vision, and of what elements are truly important to it.