Ok so my buddy is selling me his riding mower, he has had it "serviced" every year at a place on the north shore to the tune of $250. I also checked a local place near me and they charge the same amount.

It's unclear what is included in this service, but basic oil, filters, and spark plug changes will likely only take me like an hour tops and cost like $50 tops! If I do those things, will I be ok if the thing is generally running well or are there other major things I should be doing? I've already looked at all of the fuel lines and the pully on the deck, they all look good!

I think they are likely charging $100 an hour for stuff that it sounds like you can easily handle yourself. I bet they are able to bump it up a bit because they "inspect and sharpen the blades" as needed.

You should be fine doing it yourself. Very easy to maintain a riding mower.

sounds like I"m gonna go into business doing this for $199.99 :D