Good rule of thumb is about $1000 for the unit and installation. This will vary just a bit based on the hardware and wire pulls necessary. We've always had good luck with the offerings from Coleman. They have several units running 8 BTU to about 15K BTU. If the unit is only going to cool the bedroom space, a smaller unit at a lower price would probably do the trick. You may also pull fewer amps with a smaller unit, thus allowing you to run both A/C's or an A/C and more power-consuming appliances/lights at once (based on your electrical service). No reason that you shouldn't be able to install it yourself, BUT 1) if the wiring or hardware isn't in the configuration that you expect, the self-installation might become pretty tough (in our pop-up, several mods were necessary), and 2) you won't have any warranty on labor.
For current retail pricing, check Camping World at
www.campingworld.com . They have several units catalogued. We contacted a local RV dealer for our aftermarket installation. Their price was quite comparable. Good luck!!