Hopefully the head is not warped badly. It needs to be checked closely and shaved if it is warped. Putting an even slightly warped head back on an engine is a waste of money as it will blow sooner than it should. As far as cost to repair i would say in the $1000 range. Machineshop charges around 200 if they do a valve job while they are at it. As far as damage to the lower end it is not likely since it is a slight leak from your description. However water in a cylinder is bad news and can damage the cylinder walls/rings. Only way to know if it did is remove the head and inspect the cylinder walls. I doubt you need a new engine.

 

Probably in the $1000 range. Make sure the head has no warpage at all. Machine shop charges should be in the $200 range with a shave and valve job. An inspection of the cylinder walls should tell you if you have lower end damage.

 

$300-500, depending on if the head is badly warped.

Cheapest you are going to get a boneyard motor, 1.6 or 2.0 is $550. Then you have labor, questions about motor, may need new ECM. May be better off doing the the gasket, you will know about the engine health when you get it open. And get a brand new mazda radiator.

 

hope this could help,
basic automotive cooling systems
http://auto.howstuffworks.com/cooling-system.htm
mazda miata radiator information:
http://www.griffinradiator.com/faq.php
fan clutch information:
http://www.haydenauto.com/products/f...fan-blades.htm
troubleshooting information:
http://www.gtoalley.com/ttcool.html
http://www.theautochannel.com/mania/repair/cooling.html