Much cheaper relay:
If the "resistor" your mechanic referred to is the relay, then you can get it at Radio Shack for $7.
Take the old one with you and match it (there is a switching diagram on the top of the relay). It's a simple, very common relay. If you really want one that's from Porsche, get it at one of the "big 3" 928 parts sellers for $12. The part number is 141 951 253 B
www.928intl.com www.928gt.com www.devek.net Get a few for future problems. There are 8-10 of this relay in the Boxster so you will use them some day.
Another way for fans to fail:
As for the fan going out, there can be other things that cause the problem too, but it would be different from Randall's problem with both fans going out rather than just one. If the coolant temp sensor isn't working properly and does not read an operating temp condition, it will not signal the fans to come on.
Keep it on the road:
One thing I did with the rear fan that I will be doing soon with the front ones was to put a manual override switch in. I did this with the rear fan because the engine compartment temp switch died. It's a week wait for the part so I bypassed it until it gets here so the fan is on when the car is on. I put waterproof clips on so I can leave the switch in and just reconnect the fan to the system when I get the part and put it in. Then if I ever have a fan failure while I'm out somewhere, I can just clip the override switch in and turn the fan on as long as it's not the fan itself that has failed.
Debris in the front ducts:
Like Randall said, it's a great idea to clean it out, but there are other problems with the ducts you may want to head off. My PO had a rock go straight through the ducts and wack the A/C compressor killing it. PCNA replaced it, but the dealership's mechanic said he had seen several Boxsters have the same problem and suggested covers that would block big rocks but still allow the airflow. He put them on and I have not had any problems. They also keep the big leaves, etc. out so there's not much cleaning.