The budget you want to spend for your trip really depends on how you want to travel and what services you want to use. Travelling as budget travelers or backpacking is very much different from a luxurious guided trip. It is the same in Iran. But in general, in comparison to many countries, Iran is definitely cheap in many aspects. By western standards, Iran is dirt cheap in services. Even after the removal of subsidies in 2010, and despite the fact that most prices have doubled, the country is still cheap. The bus fare for long distances usually costs 10 – 22 $. (Tehran- Shiraz is about 1000 km and the bus fare is 15$). The train is much the same. It is currently 6-22 $ depending on the route and the type of train. There are still some guesthouses for less than 20 bucks. (Before this you could find some 2star hotels for 30$). So perhaps you can consider 600-1000 dollars for a solo budget trip for one month.