factory reset ,re-install i recently factory reset my phone and wiped everything off it.
i have re-installed my phone and logged back in to the play store with the same Gmail account. my question is , why is it when i look up installed apps and come to re-install them again does it say the in app perches price of 2.99 ? does this mean i have to pay for it again ? , because i thought if you buy it once you never have to buy it again if you re-install on the same account!.
No, you shouldn't have to buy the app again.  If you bought the app, you should have a receipt in your e-mail inbox or a folder you put them in.  E-mail the support team of the app in question by going to the app page on the Play Store, scrolling down, and click on "Send email", attach your receipt so they know you paid and they should either refund your money so you can buy it again, or make it so you don't have to pay again.  Just a word of warning though, some support teams aren't the best depending on what app it is, so there is an outside chance you might have to pay for it again if they don't respond.