Hi altinkumdan,
Yesiltepe is what many consider a more 'select' area. i.e it is generally villas/more expensive properties, and it is outside the resort of Altinkum. It is actually closer to Akbuk than it is to Altinkum. It is a very popular area and has quite a good mix of ex-pats from UK and Europe and turkish living there. Prices here are stable - although there has been little rise this year in prices due to the economic situation in the UK. It is still a good investment area, and properties here do tend to get a higher market value than in other areas of Didim
Bafa lake is another option, but as yet the infrastructure is not as developed as Altinkum, Akbuk etc. It is a good choice for anyone looking to get into land or property owndership with a small investment capital. you can get land here from £3,500. But it should be pointed out that a purchase here is with a long term view, and is not for those looking to cash in anytime soon. Also, until tourism rises in this area, the main resort remains more attractive as you have a wider pool of potential renters/buyers. Not everyone is into fresh water lake resorts, and at present Turkey is more known for its beach resorts. However, as I said, it is a good idea for those with minimal capital to get into overseas investment.
Hope my information has helped.