How much to fix a coolant and oil leak on a 2000 audi a6?

First of all, are you sure it is a coolant leak and not the natural draining of the Air Con., a leak on piping to or from the washers, condensation from the exhaust etc.?
Does the coolant level drop significantly and if so, how much and over what period?
Can you collect in a tray the liquid that is leaking? What does it look and smell like and what colour is it?
Does it leak with the air con disconnected?
Does the heater work properly?
When you say that it is leaking 'right under the middle', I presume you mean the middle of the car. If you park over a newspaper, where does the drip end up (which might not necessarily be immediately below the leak)?

You have a small leak on the rear of engine whioch is difficult to see. Run the engine to operating temperature. Put it on a ramp and examine from underneath with engine idling.

You have coolant burning. Your 2.7T is a hot-running engine and the plastic tank sits above the left side exhaust manifold. The heat from the manifold is known to cook the tank and make it brittle enough to crack and leak. I think if you look at the tank you'll see it's leaking slightly on the manifold, causing the coolant smoke you're seeing. You can get a new tank from www.GermanAutoParts.com in Albany NY - great place to deal with (I always order parts for jobs from them) and replacement is simple.

The oil pressure sender unit is located on the oil filter housing.These do tend to leak at times. The warning light for this switch is a red oil can and a buzzer. Replace the switch. If warning light still comes on, have the oil pressure checked. The oil level sender warning light is an orange oil can with oil level under it. This sender is fitted on the bottom of the oil sump and can leak past the seal. If warning light comes on, check oil level. If ok, check for damage to wiring under vehicle. Senders do some times become faulty.

could be a pinched gasket or filter was not screwed in tight enough

the oil light only comes on if either A) not enough oil pressure or B) not enough oil in the motor, caused by burning or a leak.

With the year difference only being two years apart I would go with the lower mileage vehicle.But remember that just because it's lower mileage doesn't mean it's in better shape mechanically.A quick trip to your trusted auto mechanic and a small charge of $25 per car can get you a prepurchase inspection, which will tell you any problems the vehicle might have, aka oil/fluid leaks, etc.I hope I could help.....get back to me with any questions

quick method is to park the car overnight, mark the wheel locations with chalk. after you have moved the car have a look at the pavement. that will give you an idea or what location the fluids are leaking from. if the water pump is failing it will start leaking water from a small "weep" hole on the front.
check your oil as well and let me know what colour it is as well as the colour of the exhaust smoke.

sounds like a faulty oil level sensor i,v replaced them in the past you need to get it checked first because the sensors aren,t cheap