We are in the need of a microcontroller with USB interface that can be
installed in one of our products. Pricing is however a major issue and
it may not cost more than $3 in larger volumes (1,000 ? 10,000 pcs).

It is also important that there is a cheap development platform, e.g.,
a reprogrammable flash-version of the microcontroller, or another low
cost (<$200) development platform.

Cypress has a microcontroller that would work, but their development
platform is quite expensive and thus not viable.

Please supply us with a list of the best and cheapest microcontrollers
that meets the above requirements.

Check out the SI Labs(Cygnal) C8051F320

http://www.silabs.com/products/microcontroller/usb.asp

Microchip's PIC microcontrolers are popular witha wealth of free (or
nearly free) tools available on the web.

Here is a link to their USB enabled chips ($2.86):
http://www.microchip.com/ParamChartSearch/chart.aspx?mid=10&lang=en&branchID=111

Here is a starter link for PIC microcontrollers.
http://www.piclist.com/techref/piclist/index.htm