I don't know the dimensions for the Ruger LCP, but those for the Kel-Tec P3AT are 5.2" by 3.5". I have to assume that the LCP is pretty similar.
The Rohrbaugh 9mm is 5.2" by 3.7".
(Eli: My AMT .45 Backup is six inches by four inches.)
Making 'em smaller makes 'em expensive to design, engineer, and manufacture. Therefore you will never find a 9mm the size of the LCP at a price like the LCP's.
There already is the Rohrbaugh, a 9mm the size of the LCP, and its very high price reflects the difficulty of making it, and assuring that it will work reliably.