There are fixed costs for the well pump, holding tank, etc...but in my area (southern New England) a 200' drilled well with all the fixin's runs about $3500. That's everything, electrical, steel casing, half-horse pump, etc. You may have a small additional cost to trench the electrics and water pipes from the house to the well head.
Septic can vary. If your perk is good and you don't need an engineered system, a 4-bedroom system will run about $2500 for a simple pipe and stone leach field to $6000 for a run of concrete galleys. An engineered system, or an alternative system (normally requires an engineer's stamp) can easily push $15000 to $20000.
Advantages? No monthly sewer or water bills. You'll need to have your septic pumped every 4-5 years at about $150 a shot.
Me? I prefer my own well and septic.
If you make an offer on the lot, make it contingent upon a successful perk test. You can ask the neighbors (if there are any) how deep their wells are, what kind of septic they have, and what their costs were. Typical drilling runs about $6-$7 per foot.
If you can have your well pounded instead of drilled, consider it. It's a better quality well (better water flow), but may be slightly more expensive.