Are there only two levels to judge the work - accept and reject? Can I post the task price as the maximum price and then pay the worker less if I am not satisfied completely by the work done?
As Neona said you can offer bonuses for excellent work. In your case, it sounds like the bonus structure should be clearly laid out in your instructions. Rejecting a lot of HITs (which harms worker accounts) will cause workers to avoid your HITs, especially the dedicated and serious workers. Word gets around quickly when a requester starts rejecting, or even worse, blocking workers. Your best bet to receive quality work is to establish your own custom qualification, which enables you to manage your own pool of workers. If someone is not performing to your standards, you can revoke their qualification and they can no longer access your tasks. The answer is yes, your only choices are accept/reject. Your HIT should be designed in a manner that will avoid having to reject work, which can be done using custom qualifications (which could require passing a test to be granted), clear instructions and communication with the workers. Thanks for posting and communicating with us!
I am glad to hear about the bonus. Now I feel I need to concentrate only on good work and genuine posts.
Any kind of instructions should be clear. Please, make the instructions clear. As a worker, that's my biggest frustration - spending (or worse yet, wasting) time trying to figure out exactly what it is a requester wants. Examples are nice, too.
You can always do a "test run" of sorts. Put out 300 HITS,for example and see what you get back. Those workers who most consistently get it right can be handpicked to work for you. All you have to do is post their worker ID numbers in the instructions as the ONLY workers who will be compensated-all others will be rejected. Problem solved and no hassle with tests and Quals and all that time consuming, confusing ruckus. Although, if your pay is really low and the tasks take so long the serious workers can't make a decent hourly wage, you'll face more challenges because only the inexperienced newbies who haven't quite figured it all out yet will be doing your tasks and the quality (no offense to newcomers) MIGHT suffer until they're more confident.