If it's your first deal and there is no bidding war or competition, $300k is probably unrealistic.
If the company that buys the script is a WGA signatory, you can probably expect WGA min - and usually plus 10% to cover agent fees. If the company is non-sig, they can pay you whatever they want to pay you - but that's usually going to be well under WGA min. The bigger the company, the more they can pay you. Disney can pay you more than some small animation house that isn't selling the Extra Special Fake Wide Screen Ultra Special Super Edition of SNOW WHITE on DVD.
I don't write animation, but there's some foggy memory in my noggin that most of it is *not* covered by WGA.
The other thing about animation is that so much of it is written in house.
- Bill