-- Well, it's a badly made, leaky, slow Chinese 30-button for $110-$115 (+ $25 shipping)
-- I don't recall any one-row accordion as low as $100 on the 'bay (except for the plastic toy ones for $20 with seven melody keys and two basses).
...yes, and if you go that route, when you actually do get a car, you'll have learned that the way to accelerate is to twist the end of the handlebars and the way to decelerate is to squeeze the levers on the handlebars, wait a minute, what handlebars? and what does the wheel do? and what are those pedals for?
One can get a car for $500. Its miles will be in six figures and its gas mileage in maybe one, and it'll probably make horrible noises, and one will learn about all the things that can go wrong with it and either how to fix them oneself or how much it costs to fix them. But in contrast to a motor scooter, one can bring home a load of groceries, be under a roof when it's raining, have a companion in the passenger seat.
From what Lily wrote, it sounds as if she understands what she'd be getting into. She's not going to get Dipper or even Stagi quality, but she'll be able to learn how to finger across three rows and have melody notes under both hands. She will have started out on the way to being a concertina player, not an accordion player.