It is true that the average cost of a private education means that there are some schools at which you can get it for less money. (Specifically in my city, I don't think there is one private school that goes for less than $7000.)
Competition is fine, as long as it's judged fairly. Jeb Bush judges schools on standardized test scores, which, in my opinion, don't show anything about intelligence. Just to get a passing grade under Jeb's system (the tests aren't very well written, by the way), teachers have to spend half the year teaching to the test. School vouchers might work if the money weren't allocated based on standardized test scores.
Also, I'd like to reiterate that I wasn't necessarily for straight up giving more money to schools. The system definitely needs to be restructured so that it works more efficiently. For instance, teachers need more control over the money in a lot of schools.
P.S. It's great that you lived in Massachusetts, where education is an important issue already. It's also a smaller state, so the system is easier to manage.