It's not like Buy.com is actually refurbishing it personally, they just buy them refurbed from the Manufacturer. It kind of depends on two things:
1. How long is the initial refurb warranty? 1 year = I'd stick with that. 90 days = good enough to find the bugs, 7-30 days = might want to go for longer.
2. How comfortable are you replacing/repairing simple parts that might be due for an upgrade anyhow by the time they go?
Overall, $40 is a decent price for at least 1 year longer of warranty. I'll assume this is a desktop, since I almost choked when I heard the warranty prices at Office Depot when I bought my laptop.