What should I be paying for a 2000 Boxster S with 100k miles?

[Perfectlap]

Never base your decision on the asking price. Base it on the mileage and maintenance history. Unless this car comes with an EXTENSIVE list of repairs typical of cars hitting the six figure mileage mark, do not walk past go.
If $10K is what you can comfortably budget give or take a few grand, then you better have garage, tools, plenty of time and an accruing pile of cash under the mattress when a big unexpected repair comes along. If you don't have these then that $10k Boxster is more like $18k Boxster. Many people buy these cars saying "I only drive occasionally" and once they get that big grin on the their face mileage rates start going up fast and deferred maintenance comes knocking on the door. Especially if you live where roads are so so and the climate has extremes of hot or cold or both.

p.s.
if car you're considering says "new IMS" make sure you ask how many miles since that repair. If the repair was recent (mileage wise) beware. Sometimes an owner will spot metail in the oil or pan and rush to do the IMS but its too late a costly engine rebuild is necessary to clear all fragments that can grenade the engine. The time to do the iMS is when the oil is clean. But definitely get the IMS done if you are unclear as to the ownership history. Sometimes there will be many owners with markedly different maintenance AND DRIVING practices. A car can start out as low mileage, once a week or month driving with only annual oil changes and then end up with an owner who drives it far more frequently and is no better about changing the oil. Basically a long period of insufficient lubrication followed by an immediate increase in usage aka prime IMS upgrade candidate.