php - price range problem -

i developing application deals commission based on price range

i have 2 text-boxes (input type="text"), 1 lower-limit, , upper-limit

everytime adds new price range, need validate available before hand or not, , whether fall in of previous price range

example valid ranges:

1-10
11-20
21-30
31-40

what should not allowed are:

(considering following range available us
1-10
11-20
21-30
31-40
)

following should not allowed
0-5
2-8
0-15
5-12

appriciated.

if available ranges predefined , known, why not replace 2 text boxes single select box <option> each range?