the code in post #9 gives 0 for 0 pages and 149 for 1, 199 for 2, etc. Post 10 was just playing around. You could even take it further, and not worry where the minimum value starts...
Code:
var prices={40:34,80:99,120:862,160:10482};
...
$( "#amount" ).val( "£" + prices[ui.value] );