There isn't an "average build cost" because everyone's projects are so different. Sure, we might all be sticking a SBC into a Z, but the condition of the motor, the condition of the car, personal goals, standards, time frames, budgets, etc. all play a huge role in the final outcome.
First you need to set a few basic project goals, and then you need to make a plan to achieve those goals.
How much power are we looking for?
How do we want it to look?
How long do we have to do it?
How much do we need to learn before we can do it?
etc.
Once you have your plan, make a parts list with prices, and get a rough estimate of time it's going to take to complete it. Now, triple the cost and double the time - this gives a more accurate picture of what it's really going to take. You're going to run into unanticipated things along the way that are going to cost you a lot of both time and money, and there are a lot of easily overseen costs that will "nickle and dime" you to death.
You can have a v8 Z for less than a grand if you're okay with sticking in a 350 and a th350 from some old van that ran, but if you're going to build something nicer it's going to cost a lot more.
It all boils down to two things...
1. desires
2. budget
Always remember the golden rule about cars:
"Fast, cheap, and reliable -- pick two!"