I have a Weed Eater brand walk-behind that I just bought this summer. The only maintenance I've done so far is change the oil, twice. I use Castrol 10W30 synthetic, which costs about $4 a quart. I change the oil about 3 times per season.
I have a 20 HP V-twin Scotts. I scrape the deck whenever it gets clogged, sharpen the blades about once a month, and change the oil a few times each year. Never needed any major repairs - knock on wood - except to replace a tire that would not stay inflated.