Hi,
I can design and send schematics to input 12VDC and output 19VDC at 300mA, exceeding your specification. If you would like me to build a prototype for you I will need to know if you require an enclosure of some sort, what type of connectors you prefer, etc. Fee for building a prototype depends on your requirements. The electronic components themselves will meet your cost limit in single unit quantity.
I have many years experience to ensure no headache for you.