Many times the CD will be passed on to Christian shows that may also be airing on those stations. These charts are used as a guage by many radio stations to see what styles are preferred by the younger listening audiences. There are many comps being created to assist in radio play but you need to ensure you are not taken advantage of in the process. First off look to see if the comp producers are doing it as a business or as a ministry. There is a big difference, trust me.
Costs vary quite a bit.
Note that different comps have different purposes. Not all of them are targeted toward radio play. If it doesn't get out there to be heard, then your investment was not a wise one. This isn't a fixed amount as prices would need to be higher for some compilations that may have fewer tracks in hopes the DJs would be more apt to play the disc all the way through. If it's done as a ministry the comp producers need to be held accountable for the monies to everyone involved in the compilation.