Firstly, I would always suggest getting at least 2 or 3 quotes so you have some comparison, but also ask how the tradesman intends to do the work, you can get more and less thorough workmen, and cheapest is not always best.
As a tradesman, I do not reply to a job post unless the client's suggested budget is more than I think I can do the job for, based on their description. We have to pay to contact you, so I don't want to be giving a quote that is higher than the suggested budget, it will be received as "bad news" and means the the client/customer relationship starts off on a bad note. I think you need to ask yourself, do you want a budget builder? The reason we chose to work directly for homeowners when we started out was that, generally, they want to do a job properly and are happy to pay for decent materials and the time taken to do a job well. We used to do a lot of work for big landlords and it was soul destroying after a while, being told to use the cheapest materials and "do a quick bodge and shut the tenant up".
For example, two of our recent jobs had suggested budgets of £6000 and £8000 respectively. Our quotes came in at £2200 and £4680, if we can beat the budget we do, it means we are more likely to get the job, and we are not interested in overcharging prospective clients!
I hope this helps, and good luck with whatever you decide to do!
Bedford Building Maintenance