I'd have him calculate how many boxes of junk food it will take to get the bike he wants.
We're lucky, here - an old man in this village collects discarded bikes (and takes them as trade-in) and also buys new bikes and bike parts, so local kids can have a bike at reasonable rates. Every school holidays he organises a workshop for kids to build their own bike from spare parts he's scrounged. Each kid has to work on his own bike, with the old man helping them and showing them how, and at the end of it if they complete a bike, it's theirs. difficult child 1 bought one of these bikes from a mate, for about $30. The old man isn't running this as a business, it's purely non-profit. So if a kid needs a new front wheel (as difficult child 3 does, after crashing his bike) this man has supplies, at wholesale, which he sells, at wholesale, and then works voluntarily to show the kid how to change the wheel himself.
We traded in an old bike for difficult child 3's new mountain bike (the one with the buckled wheel). We've promised to let the old man fix the wheel when we've paid for the damage to the car difficult child 3 rode into.
I've got my old bike (I'll never ride it again) and I'm thinking of giving it to the old man while someone else could still get the benefit of it, once they fix it up.
As a result, we have a lot of kids in the village who ride their own bikes, some of which have some unusual modifications, and a lot of kids with a fair bit of mechanical know-how when it comes to keeping their bikes in good working order.
difficult child 3 was on training wheels until he was about 10. Then one day he was able to ride, after a long practice session at his best friend's house (friend was bike riding well at 6). I worry about him out on his own on the bike because he still gets bullied (and it was running away from bullies that caused the accident). He's still very impulsive and doesn't always see the risks. The accident could have been nasty, but it has taught him a lesson. I hope.
We paid for the bike for difficult child 3 because he does need to exercise. It was a brand-new 15-speed mountain bike which we got for A$60. Although we were happy to pay for it, we weren't going to spend big bucks on something flash. We were lucky to get such a great bike. Anything more, difficult child 3 has to pay for it. A new lamp; a bell; a drink bottle holder; patch kit - his pocket money. Which he can earn, by doing some heavy gardening for me.
We 'scrounged' for bikes, using old hand-me-downs for years until difficult child 3 outgrew them and we bought this bike. The old man maintained the bikes for us until difficult child 3 was able to take over maintenance. husband & the old man have taught him what he needs (and continue to do so).
I wish for every village and every suburb in the world, an old man like ours. We love him.
Marg