I haven't lived in Saigon for a year so any info I give is still relevant just may be out dated.
It is possible to spend 500/month all in a nice furnished place and eat good food but only after you get familiar with Saigon which isn't really that easy to do if you aren't adventurous. I say expect to spend more when you first arrive and as you get more familiar with good value places to eat and do you'll drop to that amount. In time you will be able to find a nice furnished place to live for low cost. They are getting more harder to find but still possible if you really do try to look.
Every one will get the 15 dollar price as its the normal price for these girls in those locations. They don't have the mentality there to scam foreigners since they don't see many which doesn't mean it doesn't happen. Once you say 300k, they will just agree.
I travel around Saigon visually and don't go by names to much so naming streets won't be familiar to me although they are names of bigger streets. I do know Tu Xuong though and its a street where low end hookers [average/below looking girls] stand around. The last one by the description I assume is the same as the one I know. Probably the best place for the best low price looking girls. They run back and forth on mopeds there. But than again, I havent seen that place for a year so no idea if any of these places are still there or changed locations.
Theres plenty of working girls. Every girl in Vn is a working girl to some degree. Essentially there are many girls who are willing to sleep with you esp if white if they find you safe/funny/etc to make extra income or possible boyfriend.
Very unlikely you will be able to get girls at bars/clubs at that price unless they like you. They will prob laugh at you. Def not in the popular places that many expats would frequent. Again, there are locations where they are filled with locals which would work. If you are budget minded, I would just stay away from these places because the mentality behind these places are elitist. Customers who enter these places are expected to spend. Of course there are many bars/clubs in Vietnam but this is a true generalization for them however.
Here is a great website for great local and local pricing foods.
http://www.eatingsaigon.com/