I've ranted multiple times on this very topic. Most centers in my area offer special or VIP rates for league bowlers. The main problem typically being that only time to take advantage of these VIP rates is M-F before 6:00pm, when most people are at work. I've joked before that these "VIP specials" are basically like the old sign at a bar that reads "free beer tomorrow". Essentially, I have to take a day off from work or be a retiree to take advantage of these so called "benefits" that the centers tout in their league advertising.
The bottom line is that these bowling centers profit margin comes not from the league bowler looking to practice, but the casual bowlers and cosmic bowling teens/twentysomethings who are willing to pay top dollar to throw a house ball down the lanes and act like an idiot under a black light and disco ball on Friday or Saturday night.
This is the case for me anyway. I work full time and have a family. I can't typically get to a center M-F before 6:00pm when they fill up with leagues. The same goes for my brother and teammate. The best time for us to practice is usually late Friday nights after we've spent time with our families. Consequently, we battle the cosmic bowling crowd. Fortunately, we are regulars at a couple of local centers and both do their best to accomodate our desire to get some practice time in away from all the chaos. I would say that 95% of the time we are succesful at scoring a pair of lanes, away from the cosmic craziness and without the steep price. It's not always easy though. I know other regulars who are in good with the management and do their practicing in the wee hours of the morning after the center closes.
My best advice is to get to know the employees at your center who work during the hours in which you are able to practice. Talk to them, treat them well, and generally, they'll do their best to take care of you. Another strategy (if you can deal
with old wood lanes) is to find a little hole-in-the-wall, mom and pop type alley in a nearby small town. There are plenty of these places around that usually have lanes available that don't cost an arm and a leg.