Purchasing an indoor practice pitching mound is a very large investment. Most successful youth, high school, or college programs have these mounds for indoor practice. If you are fortunate enough to be in a warmer weather state you can invest this money elsewhere, however in those cold weather states it is a necessary evil! A good indoor practice mound will cost a program over $1,000, but as pitchers get older it is very critical when they are practicing pitching they use a mound.
Practicing pitching in the off-season is critical and to properly work on mechanics you need to pitch off of a mound. It is typical that at the age of 13 kids will start pitching off mounds for games. Up until that point they are throwing off of flat ground, so an indoor mound is not necessary. Once a child starts using a mound, throwing off flat ground is not nearly as effective. Simulating throwing "downhill" off the mound for practice is critical. It can cause problems with mechanics if a child use to throwing off a mound adjusts to throwing on flat ground.
If the $1,000 investment is too much there are training mounds that will be in the $300+ range. These mounds are harder to throw off of, however they do simulate "throwing downhill", which is critical.