There is no other solution. Except for buying an old bike riding it for a week and selling it again. Or maybe a bike swap, there is a section on the HUBB for it.
Once you start thinking about it a bit, the prices *are* reasonable. First of all, all these bikes come with full damage waiver insurance, in addition to the mandatory third party liability etc. Try signing up a brand new bike for such an insurance yourself, and tell your insurance the youngest rider is 18 ... and not to take into account any discounts for accident-free years, because god knows who might ride it.
Second, maintenance and alike. If you urgently want to, you can smoke a pair of tyres in a week. How much are these? How much for other regular maintenance, oil, fluids, valves, ... ? Loss of value of the bike due to increased mileage? Compensation for additional wear ("Don't be gentle, it's a rental ..."). And on top of that, the guy who rents out the bikes, probably wants to have dinner too. And yet again on top of all that, you have the likes of VAT etc.