About how much would a minor under 21 who has taken an approved motorcycle course with a 250cc used sports motorcycle valuing around 3000 have to pay for insurance in Texas?

It depends on what kind of bike you'll be riding and whether or not it's financed. Most riders pay about $200 a year, but that could be higher for you given your young age, and again, it depends on the motorcycle. If you're going to be riding the 250cc used bike, it won't be too high.