what keyboard to get for piano lesson?

i have to get a keyboard for a 10 y.o.- but i don't have a whole lot of cash.
i think $300 is my highest but all the 'good' boards seem to be above that.

is there decent keyboard below that price and is it worth it to buy it used?

There are not true weighted-action keyboards within this price range. The cheapest one probably will be the M-Audio ProKeys 88SX, which is about $400. But is semi-weighted only. It is important that the keyboard provides a touch feeling like a real piano.

On the other hand it is not so important that the size is 88 keys. If it is 76 its OK too.

I suggest you to search on e-bay for a "weighted piano". Select "search in item description".