I saw stellaris for sale on greenmangaming for 31.99, but that's the "VIP" price, and I don't know how exactly you get that. With that said, the leviathan DLC pack is also going for 8.57 on GMG as well, so those two alone would eat up your entire budget.
Unfortunately, as utopia is brand new (its not even released as of this post), you wont be finding it on sale, and because of its price point being 19.99, you're looking at a minimum investment of around 60$ to have stellaris/leviathans/utopia[/strikethrough]
Edit: Yknow, i didnt even think to look at the paradox store. NicoH is right, its actually on sale cheapest in the paradox store, currently.