Some gain can be had by plugging the outside antenna into the phone (assuming that your phone has an antenna connector). A better solution that provides far more gain is to purchase a WeBoost (formerly Wilson Sleek) amplifier
, and plug the external antenna into the booster. You will need to either provide a ground plane (a pie plate sized piece of steel) to hold the antenna if you use the one that comes with the WeBoost amp, or purchase an antenna that includes or doesn't need one such as the Wilson Trucker Antenna
or a WirEng antenna
There are systems that don't require placing your phone in a cradle (required of the linked model above) that use an outside antenna, an amplifier, and an inside antenna, but because our trailers are relatively small, it is difficult to get enough separation between antennas to provide good gain. They are more expensive, but do have the advantage that you can amplify more than one device at a time, and don't have your phone stuck in a cradle. I had one in the past and found that I had to be within 3' -5' of the inside antenna to get any gain; with my current cradle amplifier I get more gain that I could with the old system.
As to cost, I can't help. I believe mine was the first one that Reace did (back in 2011) and was told there would be no charge because I had already added every option they offered. I doubt that is still the case!