Come on Wendy...it is entirely reasonable to do so instead of letting the country be stripped to nothing and then de-regged with 20 mil pop so that you can then take it back as a c3 with 9-10 mil pop and spend even more GC getting it put back together. I know as someone who has both taken and lost big countries that when I have lost a big country it requires a large amount of time and resources to rebuild even if you are able to take it back once it has been stripped and when I have taken countries I would be more than willing to sell them back to the person (typically, except a few of you who I dislike immensely) for far less than the value of the country because of the time it takes to actually strip a country.
Calling it a ransom isn't reasonable...there are benefits to both players especially if the player who has taken the country is reasonable and is willing to sell back the country at a price below market to save the time of having to strip the assets. Your example of buying 5 slaves at a minimum of 250 GC not making sense only applies if the player doesn't intend to re-build them...because even if all that is left is 20 mil pop which is 10 mil more than the average c3, so just the pop at 5GC a piece is worth the 50GC...even assuming that nothing else is left. As far as them being re-taken again...whether he rebuilds himself or purchases back won't make a difference with that scenario...