So, you either do two searches: one through Premium ads, and one through non-premium ads and display them in that order, or you do one search but add a "order by" criteria which puts premium ones first.
We can't be specific as we have no idea how you are storing, accessing or displaying any of your information!
But, if it's SQL, then something along the lines of:
SELECT * FROM MyAds WHERE searchText LIKE '%I want this%' ORDER BY premiumRate, searchText
should help.