i dunno... those dont look like scratches to me, but i really dont know what scratches look like.... maybe someone else has a better idea of what that is though... it doesnt seem like there would be scratches in all different directions like that
the only reason is seems like a scratch is because it obviously isn't something solid, else it would show up as a dark spot. That looks like a gouge in the glass.
I'd estimate the cost of the repair anywhere from 300 dollars and up, and I think that is shooting pretty low. With a cost like that 3/4 of the cost of the camera new, it really isn't economically a sound idea, and the older your camera is the truer this becomes, because other things will eventually have problems too.
The only reason I said just ignore it is because at 300 dollars you are halfway to an upgraded camera (K100D), and really the only thing you loose is small apertures. Sure that sucks if you want everything in focus, but you can still take most pictures just fine.
You really need to decide if you want to drop a large amount of money to fix a camera that costs just slightly more than the repairs (and it honestly may cost the same if not more to repair it, depending on what they charge to replace the sensor, and the time spend to do such a delicate job.
But, if you want to get a better idea of the price, call pentax yourself, and they may need you to send it in to get the price quote, just dont be surprised if it is as expensive as the camera itself.
EDIT: were you using a brush with bristles? If so what color are the bristles? To make sure it isn't something that can come off, get a bulb blower and try to blow those blobs off.