Price depends on how complex the background is.
Adding another character to the drawing :
+3.50$ for a sketch.
+5.50$ for a digital drawing.
+6.50$ for a digital drawing with shading.
+7.50$ for a digital drawing with shading and colored outlines.
Some complex designed characters or drawing concepts may cost extra
Almost any character, being either OCs or franchise characters
Almost any pose
Not too complicated backgrounds
Porn or fetishes (slightly sexy is okay)
Extreme/realistic gore (cartoony gore is okay)
Ponies (humanized or anthro is okay)
FNAF characters (humanized or anthro is okay)
Vehicles like cars or trains (I'm not very good with these)