$300 for a tech to spend a half a day working on anything is actually pretty cheap IMO.
Prep work, labor, misc parts and pieces that might be needed, and clean up. Then there's shop overhead and possible warranty work (figured into everything). Plus, and this is definitely a factor, the knowledge to actually be capable of doing the work right has to be worth something.
I personally wouldn't spend that since I'm a DIY guy, but I don't see it as being an unfair price. I know I charge more than $300 for a half day of work in my industry. I only make a fraction of that since all of the above is a factor, but it's what I have to charge to stay in business and have a paycheck on Friday. If you aren't making money, you don't have a job, you have a hobby.
If you're not living on the edge, you're taking up too much space!