White stuff on oil dip stick

Sounds as if you may have a head or head gasket problem, go get a head check done, this is a simple and quick chemical test for co in the cooling system, all mechanics should be able to do this test, it will take about 2 min to do and if positive (the chemical agent changing colour) you will have a head or gasket problem.

Flush oil and change filter, top up coolant and only drive to get it tested and or fixed, if it is misfiring I would tow it instead.

Cost will depend on what damage is done, the head may be cracked or warped the cost will be unknown until it is inspected, you may be able to get a quote for a change over reco head, you will have to price this in your local area as I am in Australia I have no prices, most work shops will be able to quote over the phone on a change over head supplied and fitted. As for the cause, you may have had a coolant leak a faulty radiator or thermostat often an over heating event will cause a problem like this, also not using correct coolant can cause the head to corrode and cause a problem over a long time.