Melissa King -- Ex-Miss Delaware Teen USA -- Porn Only Paid $1,500

Ex-Miss Delaware Teen USA Melissa King compromised her crown for a grand total of $1,500 ... cash she earned to star in her now-famous porn video ... TMZ has learned.

Melissa denied she's the one in the XXX video. The guys behind GirlsDoPorn.com torpedoed her denial by posting a video of the 18-year-old beauty queen, reading a release form on their website ... in the clip, Melissa identifies herself by name.

What's more ... one of the guys behind GDP tells TMZ, Melissa was the one who reached out to the website to do the scene.

The GDP rep says Melissa was totally down to shoot additional scenes ... but they say her first try was frigid at best, so GDP passed.

The GDP rep says traffic was so insane the day news broke about Melissa's porno, they had to boost their server power.