Racing anything = spending. It will cost coin to race at a competitive level. Club racing is different, but at larger events where the fast guys race, I put a new set of tires on every time my car hits the track. It gets expensive with travel, but tires will be your biggest continuous expense throught the year. And 8th scale must mean take what you think you're going to spend and multiply it by 8.
You can do r/c on the cheap if you bash or club race. Just don't expect to compete at the highest level in larger races with inferior equipment unless you are extremely gifted.
To your original question about an actual number, depending on how much you race/travel to larger events, plan on spending somewhere between 1-5 grand.