Global variable custom field parameter?

I’m very new to this but really loving WordPress, I think its great, I have learnt so much already.

What I would like to do is have some kind of parameter (a price) over several pages of my site which I can update in one place and it will display all over the site. I have heard and read the about the words in the title but not exactly sure what I’m after is even called. There maybe a widget or plug-in that can do this? I would ideally like something where I can enter Price1, Price2, Price3 etc and throughout my site reference to $price1, $price2 …
From some kind of filed I will entire these prices and they will display correctly anywhere in my site where I have $price1 for example. What is this called and do any widgets or plug-ins already exist?

Thanks in advance

Dave