I think it's always worth exposing a child to music lessons if you can afford it, even if he isn't old enough to get as much out of it as he might next near. The actual experience of enjoying music and trying something new can be beneficial for learning and brain development, especially if you don't put pressure on him to 'get anywhere' with the music, and just do it for fun. Practice is still important, but not in the sense of having to keep up with others, just for his own development. If you are willing to practice regularly with him, that will be hugely beneficial. If he waits for a year or two, then he may always feel like a beginner compared to those who started earlier, and that can be hard on confidence (even if he catches up quickly! I remember still always feeling less good because others had more chance to do ensembles, were just a bit more slick at everything, and even if I worked hard, I still always felt like i'd missed out and they were always going to be better than me). You might also find that he shines in music where he doesn't in academics, and that could give him confidence and a chance to feel successful at something.
One of my early teachers wouldn't take children until they could read - mostly because she didn't think they should be trying to learn two difficult things at the same time, and also because she thought that some of those skills would help for the lessons she gave. I'm not sure she's right - I know lots of children who have learned well before they could read etc, but she was quite old fashioned, and liked if children could read the instructions and explanations in the books and do some of the simple note-reading exercises themselves. I don't think it's necessary - there were a few very basic academic skills that did help if a child is doing formal music-reading (not by any means the only way of teaching music though) - things like knowing the alphabet (first bit) in order, forwards and backwards (e.g., helps if you want to know what note is next to F, before or after), counting and some idea of cutting things into half or quarters, but really it's mostly non-academic things like listening carefully, learning to keep a steady pulse, etc. and especially personality things - persistence, co-ordination, concentration, being able to multi-task or practicing until you can, etc. Some of those skills do tend to also help with academics, so you might find that children who find school easy also take easily to music lessons, but many of the children who don't find academics easy have developed great skills in persistence and trying again and working to get things right.
The cost varies hugely by area. I think it's more like £20 for individual half hour lessons here - and really, if you can possibly do individual lessons, I think it is much easier for a teacher to adapt to a child at whatever level of ability/concentration etc they have if it's individual, so he'd likely get more out of that than a greater number of paired/group lessons. 30-minutes sounds long, but if they're a school, some of it gets taken up by getting the child to the room, getting the instrument ready, chatting, finding music books, etc, and there isn't as much time left as you think - people I know who teach 20 minute lessons are always complaining they barely get started before it's time to return the child and get the next one! There are lots of activities a teacher can do in the time, so it's not 30 minutes of the same thing, and teachers of young children are used to adapting and making games out of activities to keep concentration up.
Parental interest and practising with a child - and actually thinking about what they are practising/trying to improve it rather than just going through the motions to say it's done - are some of the biggest helps.