A mid-range electric mountain bike is a big investment that changes how you handle trails. Expect to pay about $3500 for a reliable motor and decent frame. You will save on gas and wear, but remember to budget for a solid lock and a spare tube or two.
Prices usually dip in late fall or early winter when shops clear out old inventory for the new model year. Aim to buy then, as shops are more motivated to move stock and you can often talk them down on price or bundle in accessories like pedals or a helmet.
That 0% window is a deadline, not free money. If you have a remaining balance after the 18 months, the bank will start charging interest on whatever is left, which can get expensive fast.