what does the head gasket does in a car and if and when it should go what is the cost in a Peugeot 206

It stops the compression of the engine from going into the cooling system and other places. it can vary in price if you overheat the engine and warp the head.

Its the seal between the engine block and the cylinder head. When 'blown' the compression escapes from the cylinders. Water andor oil can get into the cylinders and engine power is lost.
Cost to replace .. about £200. The cylinder head will also need skimming to get it absolutely flat.

As others have said it seals the compression in the cylinders and prevents the cooling liquid from leaking from the cooling jacket around the engine
Price for renewal could be upto £400 as the head has to be skimmed and pressure tested.

Well it seals the head to the block. When thy make a block & a head it may or not be flat. There r imperfections on the block & the head. It goes in between the head & the block & seals the block.

Over time the gaskets head & others gaskets begin to leak & must be replaced. It would depend on a few factors to determine what it would cost

Gasket price
Labor rate per hour
Example gaskets cost 100.00
Labor time is 65 per hour
Time to do the job say 7 hours = 455 plus the gasket price of 100
Grand total = 555 + sales tax

the head gasket is the actual seal between the top of the cylinder block(where the pistons are) and the cylinder head.It makes the whole thing ''gas tight'' so that you dont lose power from the expanding gas produced by the ignition.The 206 is a nice car but l dont know the cost of a new gasket.One thing is for sure,the gasket itself wo not cost too much,its the FITTING of it that will be the greatest expense.sorry l couldnt be more help,good luck!