Well 9mm is the mostl commonly used so its the cheapest. Unfortunately ammo prices keep going up a $1-2 per box each year. When I got into shoot it was $9.99 a box and now it's 14.99-$16.99 a box of 9mm now.
.40 is $3 more and .45 is $5-7 more. The good news about .45 is you shoot half as much at the range and enjoy it just as much as 9mm so it costs the same. Perhaps because I load a 1911 with 4 rounds out of 7 a clip instead of 8 for 9mm.
Stick with the 9mm. Body guard is great for Carry get a compact or full size 9mm. Compact is good for range and carry. Depends on your budget but the beretta px4 storm, glock 19, and ruger sr9c are all great compact and range guns in the $530 range