My DS11 will be in 6th grade this year. He goes to parochial school where band is offered beginning in 4th grade. Almost all the kids sign up for band the first year and he wanted to join too. He narrowed it down to clarinet and trumpet and chose trumpet. I discouraged clarinet because he has an overbite and will need braces soon, and a clarinet would be counterproductive to the braces.
Anyway, we decided to rent for the first year. We paid I think $300 for the rental and $450 for being in band/lessons for the school year. I did require a commitment from DS to stick with it the whole year and to practice 3 times per week for 20 minutes. I told him if he didn't like it, he wouldn't have to sign up again next year. Well, he complained through most of the year, but by the 4th quarter, he thought it wasn't so bad afterall. So he signed up again for the following year.
That summer, I used searchtempest.com to search a wider area of craigslist and found a very nice getzen trumpet in another state and arranged with the seller to have it shipped. He still enjoys trumpet and was invited to join honor band in 6th grade.
On a separate note, DS's best friend did not sign up for band in 4th grade. The next year, he decided he really wanted to sign up in 5th grade, but he found out he would have to start out with the 4th graders in beginning band. So, he never joined.
I would say let them try it for 1 year and rent. Many kids did not sign up again in 5th grade, but you both may regret it if you don't give them the chance to try it. I never would have thought DS11 would like band, since he hates music class (singing), but he really does enjoy it.