If financial return is your main concern, you should consult a financial advisor or put your money in a building society. The point of investing in theatre is for the fringe benefits. You will normally at least receive a pair of free tickets to the First Night and an invitation to the Bash afterwards. There you can rub shoulders with both the stars of your show, and the list of hangers on who nobody recognises, but the publicist maintains will guarantee media coverage. If you are really lucky, the back of your head will be visible (though blurred) in the background of photographs in various colour magazines for weeks afterwards.
Depending on the producer and the show, there could be a variety of other perks. You may be invited to special promotional presentations before opening night and to birthday and other parties after. You may have access to House Seats, so you can buy tickets even though the show is sold out. You may receive a goodie bag of merchandise free, or be able to buy items at a discount. At the very least your heart will swell with pride when you pass the theatre or see any reference to your show in the media.