Okay, let's see...
Commander Shepard - Gaspard Ulliel (he's a French actor, and probably not too pricey, despite him being really good, also I like the more dashing look; in a pinch the Voice Actor for the original Shepard could be brought in for voiceover)
Garrus Vakarian - Vincent Cassel (French, has been in minor roles in Hollywood movies (Gilles de Rais in "The Messenger", for example), if not available - Michael Fassbender, look him up)
Captain David Anderson - Keith David (He's not that high-priced, also the voice, also the actor. No-brainer.)
Flight Lieutenant Jeff Moreau - Seth Green (also a no-brainer, and he would probably do it for a low budget, since he's Joker, period)
Tali'Zorah vas Normandy - Ash Sroka (Let's just go with her, if we have to cast a model, we can go for example for Kristin Kreuk, who is experienced in fighting, has played some roles, and is the right age as well as physique)
Saren - Jeremy Irons (He has played Low Budget more than once, and more than once got punished by horrible directors and writers, he deserves to be a true villain again, and Saren would gain class through him)
Wrex - Michael Clarke Duncan (He ain't too busy, ain't too expensive, and hits the figure even)
Lieutenant Kaidan Alenko - Cilian Murphy (ain't too expensive, either, and could bring a depth into the role that the original Kaidan lacks in the game
Gunnery Chief Ashley Williams - Jamie Alexander or Olivia Wilde...I'm undecided. Maybe Olivia Wilde. Depends on the price tag on both.
Dr. Liara T'Soni - Gemma Arterton (is a good candidate, but she might be too expensive already, especially in our movie where Tali'Zorah is the LI. Plus Evan Rachel Wood would be the alternative, and she might be even more expensive)
Dr. Mordin Solus - Steve Buscemi (we should be indie enough for him, and honestly...is there anybody BETTER?)
Thane Krios - William Dafoe (if we can get him...please...we would even enlarge Thane's impact on the story)
Jack - Natalie Portman (might be too expensive, though, FAR too expensive, but she should have dips. If not - Katee Sackhoff, or Rooney Mara/Noomi Rapace, the American or Swedish actress of Lisbeth Salander)
Legion - Alan Tudyk (Played Sonny in I, Robot)
EDI - Tricia Helfer (original voice, and looooook at her!)
The Illusive Man - Bruce Greenwood (if Martin Sheen wants the role, he gets it. But Bruce Greenwood would be the logical and much more affordable alternative)
Dr. Chakwas - Meryl Streep (and we blow the rest of the budget on that, if we have to, Ma'am!)
Javik - Adewale Akinnuoye-Agbaje (as beforementioned)
Admiral Hackett - Sam Elliot (Sorry, but that seems to be rather no-brainer for me)
Ambassador Donnel Udina - Elya Baskin (Just searched, and I believe he could NAIL him)
Matriarch Benezia - Michelle Pfeiffer (she should be affordable by now, and she would give a class much needed for the role. Michelle and Jeremy as villains would make the audience SHAKE in fear)