If you are talkin' 17 to 19 in stop and go city driving, that's about right. My (former) SC would get in that neighborhood in city driving, and about 26 on the road if i drove it sanely around 75 mph or so.....for a year I commuted 80 miles a day over a mountain pass (on I 90 though) and i usually drove 90+ unless it was rainy or icy, and i averaged 23 driving like that.
If you are only getting 17-19 on the highway at a moderate speed, then i'd check normal tune up stuff ( plugs, wires, air filter, vacuum leaks). Don't think DIS could cause bad mileage, they usually either work or don't, but many parts stores can test them for you and give you a printout of each circuits performance. I don't think you can buy a new one for $85 though, I think it's more like $175. I fyou DO need a DIS I have a known good one (tested at NAPA with the test results printed out) that I'll sell for a reasonable price, but I don't think you need one.