My furnace is not lighting (I believe it is a Day and Night). When I turn up the thermostat, the venting fan motor comes on as usual.. followed shortly by the igniter (I can hear the "snap snap.." of the igniter), but after that, burner never comes on, so the blower never comes on etc. It has been somwhat intermittent. The other day it seem like it took a long time for the blower to come on, but just as I was about to go out in the garage to check what might be going on with the furnace it came on, so I just assumed it was my perception that it took a long time. Then last night it did not come on at all. Just the vent fan and igniter clicking for 5 minutes without the burner ever lighting. I tried it later in the evening however, and it did light up and blow warm.. but then did not light again this morning.

I need to call someone out, but wanted to know what to expect the problem might be. Would the igniter make the snapping sound if it was not working? Could it be the gas valve. Thermocouple (is there even a thermocouple in this type of system?). Does this sound like it might be a fairly expensive problem to fix?

I just don't want some repair person to give me some line to boost the cost of the repair.

Sound like you have a dirty pilot assembly. If the gas can't hit the spark its not going to light. A restriction on the pilot hood or orifice will make this happen. Not to rule out a bad gas valve(pilot side not opening all the time). It can cost as little as a service call or around $200 for the gas valve plus labor.