Since you seem to be handy, why not just buy the components for your upgrade and install them yourself. Unless the CPU is from a local maker, Dell and others provide any telephone consultation you might need much more cheaply than paying the mechanic.

Make sure you insert parts gently but firmly. One of my early embarassments was not inserting the power cord as well as I thought I had and, after an hour with the tech from Panasonic tech, he told me that either my computer was dead or disconnected. He was right.