Despite what some will tell you, a BMW is just like any other car to maintain. You change the oil every so often, same with an air filter, fuel filter, spark plugs, coolant, etc.... Certainly no magic involved! For example, here's the prices of a few common maintenance items from a large, online BMW parts retailer:
Oil Filter: $5.20
Fuel Filter: $16.81 (a 50,000+ mile item, it's a BIG filter)
Air Filter: $11.61
The 318ti does not even require synthetic motor oil, so it takes 5.3 quarts of the same Valvoline (or whatever brand) you'd put in a Chevrolet, Honda, etc.... Changing oil on a BMW is easier than most other cars made due to the oil filter sitting on "top" of the engine. It's a breeze.
The transmission fluid in the 5-speed is 1.1 quarts of plain-old Dexron III ATF that you can get almost anywhere. That's just a few bucks per quart.
The brake fluid, again, is just plain old DOT 4. Available at every parts store out there for a few bucks per pint.
A full set of quality German front brake pads is about $50.
A brand new Sachs clutch kit is about $200, but can be found for well under $150 on eBay if you are patient. That's quite a job though to replace that when the time comes. Although, 318 series clutches have commonly been known to go over 250,000. There was a recent letter in Roundel magazine and the guy had a 300,000+ mile 318ti with the original clutch.
A factory spark plug NGK/Bosch is a bit pricey at about $7 each, but there are some cheaper options like Bosch platinums. There is a spark plug boot puller tool that is clipped to the spark plug channel! The tool is right there, so ingenious.
A spark plug socket to use is also included in the tool kit in the trunk. Things are very well though out for the most part.
Power steering fluid once again is plain Dexron III ATF. Cheap.
A quality OEM serpentine belt is about $17.
Maybe more than you wanted to know, but the bottom line is, a DIY'er should have no problem doing any of this basic stuff. It really is just like any other car; in fact, it's EASIER than most cars I've worked on just because things are so well thought out, vs. just thrown together.
As for the price, hopefully someone can chime in with current data. I haven;t been in the market for a 318ti in a number of years. I do think though that the KBB/Edmunds/etc. estimate are low. The 318ti is a bit of a "cult car" and the demand is much higher than average.