I'm not sure if it applies because of the weight of the wall but I know limos have to be aligned when loaded because the front lifts and causes the alignment to change when full. (Which is the only time they're on the road.)
If you didn't have an alignment after building the wall it could change the alignment some.
Around $100 is the average around here, but many shops are crooked and will try to sell you parts you don't need. Just get it checked at another shop you trust, even if they don't align cars, bad suspensions parts are easy to check.