So there are guidelines for the cost of just about everything available online. I've referenced all kinds of articles to give me a ballpark of what my budget can provide so I have realistic ideas when I talk to vendors. Except alterations.
I understand this can vary greatly from tailor to tailor, city to city, province to province etc. Tailors and seamstresses are very much worth their prices from my experience so I'm not saying it should be cheaper. Would have been nice to be able to better estimate the actual cost though. Just got my alterations quote from my bridal store and it's 33% of my dress price. So my slightly over budget dress is now a lot over budget. I had included $300 for hemming and alterations, but my bill is $560 if everything goes right the first time.
I had a bridesmaid dress done at the same time (am in a friend's wedding party two weeks before my own wedding. I do not recommend ever doing this!) and it's costing $150 on a $350 dress to hem and tack down two pleats so it lays nicely. Previously I had purchased a used ballgown that definitely didn't suit me and had it altered hoping that would help and it cost me about $500 (which was 2/3s of the dress price). (Side note, anyone need a beautiful ready-to-wear ballgown? She's for sale!)
For anyone wondering how much to budget for alterations, go higher than you think. Given my numbers (which is a very small sample size) estimate at least 30% of your dress cost.