What is the best sims 3 ep but I only have enough money for 50 or 40 dollars?

It depends on what type of game play you prefer.
If you like having big families with children, then Generations is a must have. It adds many interaction for children and teens, such as the prom. it also adds other things, such as anniversaries, grand parenting, child care career, etc.
Seasons is also a must have in my opinion. The seasons were beautifully done, and make the game play much more realistic and adds so much cool little details in your game, such as if you're out in the cold too much you'll get cold, and you'll be asked to go inside, you can get sunburned, you can play in the little puddles when its raining. It also adds holidays which are really cool. Just like Seasons, Pets is a must have as well, and I don't think I have to explain why.
If you like unrealistic game play then Supernatural and Into the Future is perfect.
The other two EPs that I really like are Island Paradise and Late Night. The thought of having mermaids, beach parties and resort in my game is just so overwhelming. Late Night is also beautifully done and adds so much interactions and options into your game, if you prefer playing flirtatiously and like partying. Late Night also comes with the Celebrity Points/Stars system you want.
Also in my opinion University Life and Island Paradise has the best CAS items.