It is not uncommon for people to get ripped off on a daily basis when they buy cake decorating supplies and I really hope you are not one of them! Let's take a look at the options; basically these are just the Internet or the High Street stores.
It is probably no surprise to know that whether you buy from a High Street store or on the Internet, that there is a wide variety of prices for exactly the same thing. The one advantage that the High Street stores have of course is that there are no shipping costs to pay! However, please do consider how much it costs you in fuel and parking costs to get to that shop! Frequently it is even more AND you actually have to leave the house which is time consuming!
The Internet is particularly useful even if it's just to check out what's available and what you should expect to pay. If you go searching in the shops, how long will it take to find what you want and will it be in stock anyway? How often have you gone shopping without phoning first to check stock levels? Many people are guilty of that one!!
There are other problems associated with going to the shops of course such as the weather (will you get drenched or not)? Do you have to take children with you, if so how much extra is that going to cost you in treats!? How long does it take to get to the stores? Fine if it's just down the road, but often specialist shops may not be that close. Although most of the cake decorating supplies you are liable to buy may not be large, if you get a lot, have you considered how you will carry them back home? Not that bad if you go by car, but public transport - a nightmare! Get the post-man involved, much easier and it keeps someone in work as well!
Generally, surf the web, find what you want and then do a price check at other online stores.