Your question requires some context. WHAT are you getting for that price? Hard top? Convertible? 440? Hemi? 6bbl? Transmission? Numbers matching? With or without original drive train? Options? Color? Desired result with the car? Total amount of body work and mechanical work needed?
A $3,000 car will require a lot of work regardless of whether it's an R/T or not. $5-7k is not enough of a bump to make a difference in value, quality or ease of restoration. Realistically, you will need to be over $10k, and possibly closer to $15k, for a drivable 440 automatic with decent sheet metal. You will need to spend $*** to get it safely street able so you can drive it while you spend xx years sorting out the rest.
Add 10-20% more for a four speed depending on the shape of the rest of the car. Even a rough convertible could be be mid to high teens or over 20, again depending on the total package.
The year makes a difference. Plan on spending more for a 70 than you would a 68. The 70 styling is a love or hate thing. 1970 R/T specific parts will be harder to find than 68 or 69 due to lower production numbers. Buy the most complete car you can if you want a 1970.
My 2 cents....