I think it's at least 10 years away and that's only if the league expands in which case the cost would probably be at least $1 billion - $500 - $700 million expansion fee and $500 million for a stadium.
There aren't any teams which will be relocating anytime soon. Yes, Oakland and Tampa Bay have stadium and attendance issues however, they still make money thanks to the revenue sharing enjoyed in MLB. Both teams are pretty successful on the field given their limited resources (Rays have made the playoffs 4 of the last 6 seasons and have the best manager and arguably the best pitching staff in the AL while the A's have made the playoffs 7 of the last 14 seasons and have the best GM in all of baseball) which prevents their attendance from going into the crapper like less than 10,000 per game on average.
Then you have the issue of leases. NOt sure how long Oakland's lease is, but Tampa's is extended to 2027. So if the Expos are to come back by relocation you are probably looking at more than 10 years, probably closer to 15.
I hope it happens though and am willing to be patient. This country needs more pro sports teams in the big-4 outside of hockey!