Hi, to anyone else reading, previously my post had mentioned getting Ethernet cards in all Macs capable of it. Hence itsvince's response.
Rare, kinda. Expensive, depends on who is selling. I picked mine up for a very fair price from a user here on the forums. Also, I've seen some for the SE on eBay for reasonable prices. That said, the Ethernet card for the SE/30 (which is a different card than used by the SE) does command quite the premium price.
That said, the card I got has the standard RJ-45 connector we know and love today, as well as coaxial and AAUI. Not all cards have it, but it seems a decent number did have the modern RJ-45 connector.