I hope both albums sell at least a million or two copies this year!

He's sold over 4M units, but I think some of those are singles on places like iTunes, not just albums.

Based on the articles from Billboard, here are some rough guesses in the US:

The Very Best of Prince: 100K + 216K + 55K = 371K

Purple Rain: 62K + 133K + 60K = 255K

The Hits/The B-Sides: 24K + 56K + ? = at least 80K

Ultimate: 9K + 37K + ? = at least 46K

1999: 13K + 33K + ? = at least 46K

For the most recent week, The Hits/The B-Sides is 29th this week in album sales, Ultimate is 22nd and 1999 is 8th.

feeluupp would be able to give better figures.

Sources:

http://www.billboard.com/articles/columns/chart-beat/7350372/beyonce-earns-sixth-no-1-album-on-billboard-200-chart-with-lemonade

http://www.billboard.com/articles/columns/chart-beat/7358025/drake-views-debuts-at-no-1-on-billboard-200-charts-sets

http://www.billboard.com/charts/top-album-sales

aS LONG AS IT COUNTS - THE itunes STUFF. i WANNA SEE tvbop GO mULTIPLATINUM

Taken from comments at Billboard:

Pure album sales (without TEA+SEA):
1.( -) Drake - "Views" - 851.6k (851.6k) (NEW)
2.( 1) Beyonce - "Lemonade" - 196.2k (681.4k) (-60%)
3.( 3) Prince - "Purple Rain" - 60.3k (3,362.3k) (-55%)
4.( 2) Prince - "The Very Best of" - 55.4k (2,474.1k) (-74%)
5.( -) Rob Zombie - "The Electronic..." - 40.1k (40.1k) (NEW)
6.( -) Various - "NOW 58" - 37.4k (37.4k) (NEW)
7.( 7) Chris Stapleton - "Traveller" - 24.8k (1,278.0k) (-1%)
8.( 6) Prince - "1999" - 23.6k (69.5k) (-32%)
9.(11) Adele - "25" - 22.1k (8,680.2k) (+18%)
10.(-) Sixx:A.M. - "Prayers for the Damned" - 18.4k (18.4k) (NEW)

I'm guessing the 69.5k total for 1999 is just for the last 3 weeks, while the 3.3M and 2.4M for Purple Rain and The Very Best of Prince are total sales during the Soundscan era.

More comments from Billboard:

Came across some Prince albums sales numbers that are up to date (from 1991 to 05/05/2016), if you are interested:
The Very Best Of - 2,473,611
Purple Rain - 3,362,603
The Hits/The B Sides - 793,343
Ultimate - 338,197
1999 - 862,770
Sign 'O' The Times - 348,487
Controversy - 258,240
Dirty Mind - 180,245
Prince - 268,446
Around The World In A Day - 228,292
Parade (Under The Cherry Moon) - 290,143
Art Official Age - 164,498
Diamonds & Pearls - 2,478,345
Musicology - 2,049,000
The Hits 2 - 1,738,000
The Hits 1 - 1,451,000