Needed some cleaning up and a little tuning when bought - not expensive, just a little time investment.
Broken alternator belt ~$100 in labor + $10 part.
Brake master cylinder failure. The replacement part ran about $180 and the labor was between $100 and $150.
Needed full tune-up. Approximately $300 in parts/labor for a complete, thorough job.
Oil cooler failure causing hot running (although not bad) and oil leaking. The new cooler ran $165 (a very good price when compared to other air-cooled Porsches). The labor was rather expensive, bringing the total repair bill to between $500 and $600.
The Porsche 914 is a very good entry level sports car. While it is somewhat underpowered, it's mid-engined layout allows for incredible road-handling. It can be a somewhat expensive car to repair when not properly maintained, and I would not recommend it to anyone who wants an economy ride that won't need any real maintenence or repair. If you are willing to spend a little money, I've found that the cost is worth the reward if you are a true sports car lover.
Personally, I have loved the car. There's nothing better than driving down a windy country road with the wind in your face (yes, all 914's have removable targa tops). While the cost of repairs has been a little steep, the car is now in far superior shape to when I bought it, and for the first time since I have owned it, it is trip-worthy and has no necessary repairs on the horizon.
Once the car is in generally good condition, it doesn't have to be an expensive car. If they are well cared for, 914's will run for a long time without major cost -- the key is to make the smaller investments necessary to avoid the great expenses that can arise. Additionally, the car's fuel efficiency is very good (~20mpg/town ~30mpg/highway), and insurance isn't bad either.
One very strong recommendation is to find the best Porsche shop that you can -- this is very important. Ask shops if they are experienced with 914's -- just because they service 911's doesn't mean that they see a lot of 914's. The 914 is a very difficult car to service, and in the hands of someone who doesn't have a VERY good idea what they are doing, problems can arise.