Usually if it goes on sale within a month, they will refund you the difference, if that makes any difference.
As for the tailor, it depends on what you're looking for. In-store tailors are good for the usual alterations - hemming, shortening the sleeves, etc. and those services are usually complimentary. They are also good in that if they mess up, the store usually has a policy.
On the other hand, it may be worth it to take it to an outside tailor with whom you have a good relationship etc. They may charge for certain services that in-store may not, but it MIGHT be worth it in terms of quality.
