I once wanted to make a more powerful workshop, and here's what i got:
Factory
6$, Action
All cards (including cards on players hands) cost 1$ less this turn, but not less than 0$. Gain two cards that together cost up to 4$, putting one of them on top of your deck.
It turned out good for 6$ as far as my playtesting went. I'd like to compare your Big Workshop to it. With Factory, you can gain a 5$ and a 1$, or a 4$ and a 2$, or two 3$s. Your Big Workshop always can gain two 4$s, whith a little less flexibility and no top-decking, also lacking the cost-reduction, but better avarage cost for 1$ less. I guess it's okay priced. You can always get some Silvers with it.
Edit: Forcing you to get two different cards would probably be a really strong nerf, as you don't always find two cards for 4$ or less you want in your deck. Of course, there's Silver + X, but even Silver is unwanted sometimes...