Not only does it give the appearance of someone being unsophisticated, naïve and lacking confidence, it puts the onus on me to figure out approximately how much work will be involved and what the fair market rate for that would be. It would put me in a totally awkward spot and would be a lot more headache then simply knowing that a person will do X work for X dollars.
In short, this has the appearance of being a gimmick. Basically it says to me “I don’t really know how much to charge for this, so I am going to put the onus on you to decide and hope that you won’t want to appear stingy and so will pay me more than I feel I could ask for without being embarrassed.”
A true professional knows their value and the value of the services they provide, and is not shy about making it known. In fact, that is part of the service they provide – estimating is a skill.
Maybe it is unfair, but if someone pitched this to me, I would immediately think they were fairly junior (and somewhat desperate) rather than a professional, even if I had worked with them before. It tells me you don’t have anything else on the go right now, because if you did, why would you take the risk to work for an unknown reward?
Just my thoughts…I would really think this through before adopting it as an approach.