We are at about $11,000/year for Level 6 training 7. This is an estimate, of course. It is mostly tuition costs (almost $9000), but also includes meet fees (approximately $1000) and other expenses (such as competition leotard, warm ups, travel expenses, etc.) We are in a big city so probably higher tuition to offset higher city costs -- rent, salaries, etc. On the plus side, big metro area means there are an abundance of great meets within driving distance so we don't have a lot of expensive travel (airfare, hotel).
By comparison, one of my teenage boys is very involved in year-round baseball (multiple teams too). My husband keeps track of these costs more than I do, but I'm gonna guess we are are at about half the cost of the gymnastics. Worth noting that a decent baseball bat is gonna run you about as much as a competition leotard ($200-$300) and needs to be replaced about as frequently!
For me, however, the cost is the least of it. (I'm lucky for that, I know.) I find myself getting most tired with the time commitment -- time that takes away from family, friends, etc. I literally spend hours driving to/from the gym weekly, which means less time with my older kids. (Thankfully I don't have little ones who are strapped into the car seat and stuck in city traffic with me!) Even if my DD doesn't mind so much, I sometimes feel bad that she doesn't get to do as much "normal" kid stuff like play dates after school, etc. because she is always at practice. (I've seen a t-shirt that says "I can't, I have gymnastics..." which is the truth). I've watch the number of school friends and birthday party invites dwindle.
Here's the thing: Whether it is the cost or time or whatever, once you make the decision to do it, you can't hold it over the gymnast's head. I've seen that happen at our current gym in a few instances (along the lines of: "we sacrifice so much to pay for your gymnastics, you had better do well") and it's rough. These are often the parents who are scrutinizing every practice/performance/etc. It eventually makes for an unhappy gymnast, and unhappy gym parent.
The good news: You are still several years away from all this doom and gloom! And preschoolers can be fickle. Take this information with a grain of salt for now, and see what happens. Have fun!