It will depend on the quality of aluminium. Some are so flimsy that they bend and become misshapen easily. There are different quality glass as well.
The best thing is to try and find the company name of the greenhouse and glazing type then look up how much it would cost new. An estimate of the price can then be decided by age, condition etc.
We decided on a polytunnel after extensive window shopping and googling greenhouses last year. Just erected this year so don't know how well it is going to do compared to a greenhouse, but here's hoping it stands up to the winter gales