Tricky Group By Order By Query

Hello, I'm trying to find the most optimal way to perform a tricky
query. I'm hoping this is some sort of standard problem that has been
solved before, but I'm not finding anything too useful so far. I have
a solution that works (using subqueries), but is pretty slow.

Assume I have two tables:
[Item]
ItemID int (Primary Key)
ItemSourceID int
ItemUniversalKey uniqueidentifier
Price int

[Source]
ItemSourceID
Priority

I'm looking for a set of ItemIDs that match a query to the Price
(something like Price < 30), with a unique ItemUniversalKey, taking the
first item with each key according to Source.Priority.

So, given Item rows like this:
1 2 [key_one] 15
2 2 [key_two] 25
3 1 [key_one] 15

and Source rows like this:
1 1
2 2

I want results like this:
2 2 [key_two] 25
3 1 [key_one] 15

Row 1 in Item would be eliminated because it shares an ItemUniversalKey
with row 3, and row 3's Source.Priority is lower than row 1.

Help!?