How much should it costs to fix an antifreeze leak on a 2008 Mitsubishi Outlander XLS V6?

So you want a guess do you?

Bad hose clamp under $70

Bad head gaskets $1800

Does that make you feel any better?

Get the cooling system pressure tested and find the leak so you know where the leak is.

LEAKING WATER PUMP???  drive over an OIL CHANGE CTR PIT and they can SEE the leak from UNDER THE CAR,  Then use ROCK AUTO fr or replacement parts .  YOU MUST use ONLY genuine MITSI COOLANT and not PRESTONE or PEAK! There is NO SUCH THING as UNIVERSAL COOLANTS just like there is NO SUCH THING as UNIVERSAL OILS either! EACH CAR TYPE has Specific NEEDS for OILS and FLUIDS these days! AN oil OKAY in a JAP CAR may NOT be GOOD in a EURO or DOMESTIC CAR! That is why TOTAL ENERGY OI makes THREE types of OIL with the SAME EXACT viscosity ratings!  use if THEY topped it up with the WRONG c FLUIDS I won MANY Times at THAT issue! IF the engine was EVER overheated, you may be dealing with a LEAKING HEAD GASKET! The CHEAP CURE is top replace the HEAD BOLTS o for about $35 for PARTS! If it IS the HEAD GASKET you will need NEW BOLTS anyways! >>>CAHNGE the COOLING FAN SWITCH as a MATTER of AGE and COURSE! Th3y DO fail with time and it MAY cause an engine to OVERHEAT! USING PENNZOIL ULTRA PLATINUM OIL will get you a WARRANTY out to 500,000 MILES,  AND there are ADDITIVES you can use to STOP some types of LEAKS!

All good answers, but to add to support, you might even have a bad waterpump.  If you're losing coolant and you let it go too long, you could end up blowing the headgasket. Is your car due for a timing belt service? Have mechanic check it out. Better yet, have a mobile mechanic check it out. They will come to you and will be able to test your cooling system for generally less than a shop would charge. If it's the waterpump, it might be time for a timing belt service. That will include replacing seals crankcase/ cam seals, water pump, and replace all belt. Since it's the Mitsubishi 3.0 ;ioter V6, I would recommend replacing the hydraulic tensioner while you're at. It will save you a lot of time, money and headaches if you do that with each timing belt service.  

It depends where it's leaking from. If it's a hose leaking that's a really easy, cheap, fix. If it's the water pump seal that's started leaking that's a pretty decent job that a mechanic should do unless you've done it before and know what you're doing.

It's not extremely difficult to do but if you don't know what you're doing you could end up taking a bath in anti-freeze, and not putting the seal in exactly how it needs to be could leave you with the same problem but worse than it is now.

This needs fixed fairly soon before it gets worse. If you're losing anti-freeze that means the engine is losing coolant. It could overheat which can cause a heck of a lot more damage that's way more expensive to fix.

If it's a hose that might be a total of a $100 to get it fixed. I've had water pump seals replaced on 2006, 2008, 2012, and 2016 jeeps and the total for the repair was less than $250.

Now if the water pump is going bad that's when this can get up and above possibly being affordable to fix for some people. It's probably not the pump. It's most likely a hose leaking, a hose clamp that isn't holding the hose on (which is a 30 second fix with a screw driver or replacing a $5 clamp, some clamps require pliers to get them off and on), or the water pump seal.

Get this looked at as soon as possible, and fixed within a week or 2 before the leak starts getting worse and it starts causing the engine to overheat.

And don't keep spending money on anti-freeze until you get it fixed. You can use plain ole tap water to keep capping it off as long as you're not in a cold weather climate that would freeze the water.

First, find out where the coolant is going. If you have a gasket leak, coolant could be getting into combustion chambers and getting blown out through the exhaust.

The cost is directly related to what is wrong with the cooling system in the first place.  If it's something simple as just an easily accessible loose hose clamp, the cost can be free.  If it's a bad head gasket, the cost rises significantly...

Replace the radiator cap and see if it fixes the problem.  No luck, see the $80 an hour dude.

are you overfilling it ? its not a coolant tank ..its an expansion tank ..its not meant to be full 

 It should cost as much as someone will charge you to fix whatever is leaking

The car is over ten years old. I would go down to an auto store. First, try buying a new radiator cap. That may the only reason you are losing fluid. If that does not fix it, buy some leak stop for the radiator. You just dump it in and it can seal the leak. That might fix the problem. If that does not work, check about swapping out the radiator for a new one or a used one from a wrecking yard. I would do all this before I took it into a dealer and told them to fix it.