Pretty sure switches that support VLANs are more expensive than a NIC. I think even a 4 port GigE Intel NIC can be had for less.

But you might want VLANs anyway, so it's an interesting thing to consider.

VLANs are fine. Running your whole core over one trunk into a general-purpose box gets dumb fast, because one bad config or L2 loop turns into a host-side debugging session.

Extra NICs move forwarding work into the host, and you pay for that in CPU time. If you care about isolation and wire-speed, buy a cheap managed switch instead of stuffing more NICs into the box.

Netgear GS305e supports VLANs and can be ~$21