This depends on a lot of things. What kind of car it is, what kind of transmission, what seal or gasket is bad, give us something to work with here.
It could be leaking from a number of places. Some could be a simple fix and others not so simple. Need more info as to where it is leaking.
The cost of the repair of a transmission depends on the what is wrong with it. It could range from about $100 to several thousand if it needs to be replaced.
1200 $ for a new head and instalation
it depends on where its coming from
I had my transmission rebuilt on my 2004 Mountaineer. I believe it cost around $1800
I had my S type done 1 year ago it cost me £450.00
how can i check the transmmission fluid?
It would depend greatly as to where and why it is leaking.
Replaced the pan gasket with a new one.
Fix the cause, or keep adding fluid.
check your radiator hoses and change gaskets
Depends on where it is leaking. If it is leaking at the pan gasket, then try tightening the bolts to 20 lb/ft torque. If that doesn't work the gasket will need replacing. This is a good time to have the fluid and filter changed. If it is leaking at the seals then you can try a can of some sort of transmission stop leak. Any Auto Parts store can recommend one. May or may not stop the leak. If the leak persists and is more than a few drops a day, the transmission will need repair by a pro.