Adding attributes

[vpervades]

Hi ,
I have created a attribute a textbox .
How I have set the price for textbox attribute is $100.
whatever customer enters in that text box ,it only shows that $100,But I need to make it in such a way that -
if customer enters 1 in the textbox attribute ,then it adds $100 .
if customer enters 2 then it adds $200 .
if customer enters 3 then it adds $300 .
if customer enters N then it adds $N00 .
Ie instead of displaying the price direclty ,i need to multiple this attribute price with this attribute value.

Is anybody done this?
any idea how to do this?
Any help will be very useful.

PS:
I have tried this with the dropdown - But i need to set price for each option ,it is really not possible and it also RESTRICT the MAX. number.

So ,Please advise me to accomplish this with text box.

You can add a text box attribute but it will not validate the number of items - - A human will have to insure that it is correct