Well, the cheapest will be the Jubilee or Aristocrat and very easy to work on; very reliable too. I've picked one of these up for £75.00 on ebay. You see these sell between £130 to £250.
The Sega's many different varieties, I own two: Sega Windsor, £110 paid - lovely little machine with small footprint. A Sega Mad Money type machine, paid £70 for... I do not recommend the Sega Continental - very very heavy machine and a bit of a dog to work on and restore. Lots of metal, too much for my liking. I had one, renovated it and moved it on for a tidy profit.
Mills, very desirable over here; I would like one but have been outbid every time; I'm not prepared to pay £700+ for a High Top. If one comes along cheap then I will be lucky.
Jennings, once again, expensive.