On average how much does alterations cost?

This will vary by the style of your dress, your location, and specific alteration shops. Alterations for my a-line organza gown will be about $450. I would certainly budget for at least $500. But, the only way to know is once you choose your dress and then reach out to alteration shops for quotes.

Unfortunately alterations cost vary considerably woth different dress styles. There's too many variables to blindly give a ballpark number.
Most seamstresses will do a free consult, where they can look at your dress and give you an accurate quote.

I was told when I ordered my a-line satin gown alterations will be around $600.

Mine were $795 for hem, a complicated bustle, taking it in several times, and adding cups to the bust. My dress was all-over lace and beading.

Mine were $400 with an $100 tip

Mine are $650 for a mikdao trumpet gown (no lace, no beading).

650 for hem, bustle, taking in the sides a little, adding cups.

Hi Danielle! There was a great post about this a few days ago, you should check out those responses as well:

How much did your alterations cost? !

Mine came to about $525 I believe, but my shop capped the cost at $500 so that's what we paid

Mine was $400, however, hemming is the most expensive aspect of alterations I've heard. Mine came with a customized hem from the designer - the bridal store pays for it. I am getting a bustle, the bust will be taken in, boning taken out and some other little things...my dress was lace and mermaid.

I'm getting mine done by someone for around $100, but the salon I got my dress at said the range was $400-$700 depending on what needed to be done, which I refused to pay (what I need is fairly simple).

I got mine shortened, straps altered, cups sewn in, taken in sides, bust reworked and a bustle for 550. I thought this was a great price.

My alterations will be 200 total when done. I shopped around for a bit before i decided on who to take mine to. Im having the zipper taken out corset put in. Pleating taken out. Seams let out and fixed sash removed and replaced. If i would have wanted the bustle and the train off it would have been 300 more. I didnt need a hem at all.