To really give you an acurate property value, an agent will need to visit your property, know the area well and run comps. You'll find these posts helpful:
The cost of dirt in Preston Hollow
How 'busy street' proximity affects value
Both of these posts pertain to your neighborhood and situation.
Please, let me know if I can help you find a buyer.
By "knockdown" I assume you mean you are selling the house for lot value. There are many factors when pricing a lot. Backing to the Tollway may not be as bad as you are thinking. It really depends on the shape, trees, accessibility, privacy, & "buildability" of the lot in question. Just from your description, I think I know where you are. I'll drive by this evening and take a look. If you want, we can set a time to talk about your options. You can reach me by email or phone, whichever you prefer.
I do know that MANY developers are looking to get into the area. Lots are still selling. Not at the pace they were a couple of years ago, but they are definitely still selling.