estimated cost for parts?
the most expensive part by far is the new tuner, which you can sometimes find on ebay for around $200. it can sometimes be difficult to get the service center to sell you the tuner, since it doesn't apply to your car. i got a quote from them before buying it on ebay, direct from tesla it's $225 - at least it was last year when i first did this mod.
if you want a second model 3 tuner to be able to use that for the housing swap and keep your existing tuner intact, that's another $50 or so on ebay. don't get that from tesla, they charge like $600 for the tuner brand new (i remember this one because it was so absurd).
the rest of the parts needed (antenna and adapters) are cheap.
rough ballpark estimate... absolute minimum (if you want to leave FM completely disconnected) is somewhere in the neighborhood of $250. that's the new tuner, the SXM antenna, and the SXM antenna adapter.
if you want to connect only one side of the FM antenna and live with degraded reception, you're probably looking at an additional $25 on top of that.
if you want to connect everything, including both sides of the FM antenna (which gives you basically stock FM reception plus SXM), add another $35 or so for the additional parts needed for that.
so, somewhere between $250 and $350 (allowing for a bit of overages if some of these parts cost a bit more at any given time) is a good rough estimate. you can also then add another $50 for the spare model 3 tuner on top of that, if you want to go that route.
cost for the model Y will probably vary slightly, but this is a good rough estimate for both.