I, for one, am a little confused. In your first post you say "I need to have an ethernet-RS232 converter for interfacing the card to software." To me, this implies that you have a board that has a serial interface and you do not wish to make modifications to this board, but you want to have a standalone "black box" to interface the device to Ethernet. However, now I am getting the impression that you do have some flexibility to design Ethernet hardware and some software for Internet protocols onto your board. Can you clarify for us which it is?

Certainly some of the higher costs for the standalone "converter" devices are because they are just that -- standalone, requiring separate micro, memory, power supply, case, packaging, etc. If you can put an Ethernet interface directly on your board, you obviously avoid all that per-unit expense. Somewhere in this thread you've been given a link to IiChip (or something to that effect) that has much of the Ethernet/Internet stuff built in (and yes, I know the two are not the same).

I have not gotten around to downloading the appnotes and code, but there's a bunch of (free???) 8051-Internet at http://www.atmel.com/products/8051/atweb.asp