You're running an A-B Ethernet driver right now, probably in RSLinx Professional. The 1761-NET-ENI translates between the A-B Ethernet protocol and the A-B serial protocol (called DF1). No other device on the market does that, so there is no direct replacement for the functionality you have now.
What virtually all RS232/Ethernet devices do instead is make use of a "Port Redirector". This is a software element that would reside on your webserver computer and emulates an enumerated COM port, like COM4 or COM5 (up to COM31 in Windows). It intercepts calls to that COM port and redirects them across TCP/IP Ethernet to the serial port device.
This means you'd have to change over to a DF1 driver in RSLinx, and you'd be limited to 31 of them.
I prefer Lantronix, because their quality and support is good. You may be disappointed with the ease-of-use, though; there are a handful of threads on this Forum that detail users spending several days trying to set up these inexpensive interfaces.
All in all, I think the Net-ENI is worth the money, as that money pays my rent.