The impeller and gaskets should not run you over $35. They are easy to install yourself. The hardest thing is getting the shifting linkage and speedo tube back in place. If it is over 2 years that you had it replaced I would spend the money now. Mercs idle at about 5 to 7psi. Have you thought about installing a water pressure gauge? They are not that expensive and you can also tell the thermostats and poppet valve are working properly. If you fish around weeds, it could be the cheapest item to have on the boat.