Nope, bad plan.
Let me tell you why.

1) Most wild rodents have a sworm of possible diseases and parasites which could spread to your snake and kill it.

2) Rat poison is so common in areas with lots of rats, its more than likely at least 1/3 of all the rats you see have some trace of rat poison on them!

3) If your doing this to save money, likely one way or another it's going to cost you hundreds of dollars trying to help your snake when it gets sick. It is NOT worth the cost.

When you buy a snake, these type of things need to be taken into consideration as well. If you have trouble paying for rats, try setting up a few (petstore bought) to breed and it could save you more money any day.
If your snake eats pre-killed, I assume it eats frozen too! You can buy frozen online in bulk for very cheap prices compared to what you would have to pay at a petstore or even to breed.

:]