if your current rig got space and PSU has sufficient Watt headroom to run another 3090, can just use your existing one.
to build another rig just for one gpu, you have to factor in MB,CPU,RAM,Storage,PSU,extra cooling fans,case/frame cost. not to mention the additional watts consumed (thou not much) just to power it for one gpu.
can consider carousell used old parts. the cheaper bundle the better. ITX/matx/ATX MB all also can since just one gpu don't need to buy risers.