There are two ways of answering your question, depending on whether you are buying your house to live in or not. If it is pure investment, then first you can rent the property out and second there is the capitalization of the property in the time. The rent depends on the size of the chosen house of course.
We are now in the begining of building the infrastructure. The project is considered to be ready july 2008. Till then we ourselves are going to increase the prices in stages. For this year it will be around 20% exceed. Annual growth in general is a result from the mix of location plus concept, competition and key costumers that are in the project. We did it in our first project Sofia Gardens, where it was again a complex but with apartment buildings. You may visit the site of Sofia Gardens for more information - unfortunately I cannt post the direct link to it.
Quoting statistics about the real estate market in Bulgaria "Return on investment (yield) for the first 6 months of 2007 vary between 7 - 10 %".
I hope this information answers your question. Should you have more questions, feel free to contact me again.