(not too sure about exact amount for MS points)

Dawnguard is 1600 MS points.
Hearthfire will be 400 MS points.

1000 MS points = About $14.50
500 MS points = About $7.25

Dawnguard and Hearthfire prices in points make up 2000 MS points.

$14.50 +$7.25 + $7.25 = $29


TL;DR Both X360 DLCs are going to be around $29.

According to http://www.360plex.com/points_converter.php converter:
The cost in Dollars for 2000 Microsoft Points is: 25.00

I don't play on 360 but I know most DLCs cost 1200 MS Points ($15), there 1600 MS + 400 MS = 2000 MS ($25)

First of all... There are no packages with 1000, or 500 MS points. There are packages for 400, and 900. 400 MS points is $5.00 (I'm rounding up.) and 900 is $10.00 (rounding up again.) and 1600 MS points is $20.00 Dawnguard is 1600 points, hearthfire, 400 points. So please, remove $4 from your concluded price. It's $25 my friend.

800* not 900.

And everyone, $1= 80 MS points.
Dawnguard= 1600 points ($20)
Hearthfire= 400 points ($5)

$25....

So is Shivering Isles worth one Dawnguard and two Hearthfires?

ha no offense but your math svcks

1600 ms points = $20

1600 divided by 4 = 400

$20 divided by 4 = $5

dawnguard + hearthfire = $25

$25 well spent IMO, I can't get enough Skyrim! The amount of content in Dawnguard alone rivals many full price games.