It totally depends what you want a mower for when you buy it - if you want brand new- then research and buy new- new ones even have issues as well, keep that in mind.
Personally i wont spend over $50 for a tractor, altho ive gone as high as $100 - thats not counting the one brand new White MTD i bot in '92 tho ( which has given me some issues, but expected tho after 19 years).
To me it doesnt matter how rusty or broken a machine is, its either 2 things - parts for my other tractors or rebuild it to mow with. Cheeper the better - actually my free to $25 ones have been the best tractors, some have needed a ton of work- but now they mow flawlessly.
Thing that i check first is if the motor runs, if it does- then its instantly worth it, wether the rest of tractor is in good shape or not -a running motor is worth $25 - 50 , then spend hundreds on a new one.
Ive had motors full of rusty water that run perfectly fine after cleaning up- also had perfectly good looking motors that are nearly worthless because of internal damage, only thing of any worth are external components, like starter, carb, ect.
Just depends how dirty you want to get your hands when buying a new mower, spend a ton of money for a brand new machine that can have potential issues and headaches or a mere $50 on a machine that needs work, but you know will run good after being rebuilt/gone thru.