Many websites compare car rental prices.
Are there any such websites for bicycle rental?
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It certainly exists for the USA:
RentaBikeNow.com help cyclists find and reserve rental bikes 24/7 Shop, it allows you to compare and reserve items from participating bike shops when it’s convenient for you. Reservations can be made at more than 250 bike shops.
There is also at least one site where you can view listings for bicycle rentals from private citizens (includes prices):