Low Cost Music and Opera Performances
If you have a university in your area, most music departments have at least one major musical or opera per year. These events are nominally in cost (usually free or between $5-10). Proceeds typically go towards the cost of the production or music scholarships. You have an opportunity to enjoy a great stage production as well as support the local music department.
Some high schools will also host musicals (especially now that musicals are so popular for the teen group). These are less formal (since everyone's little brothers and sisters will be in the audience). And they are usually low cost.
If having young children is an issue, you might want to contact the music department or even local opera company and ask if there are any educational programs for children (usually these are reduced or free) or if you can attend a rehearsal with the children. Most large scale professional ensembles have an educational program that is open to the public or to children (there might be age limits). These are low cost or free.
The local university may allow you to visit during dress rehearsal (when all the costuming and staging is complete) as long as the children can be relatively quiet. The benefit of attending a rehearsal (besides not taking place at night) is that you can leave whenever you wish, have bathroom breaks, and the children do not have to stay put in one seat the whole time.
You can enjoy amazing musical performances with a little ingenuity and little money.