I think it should depend upon the type stock in question. If we're going to do stocks in Shadowrun, there's no reason not to apply intricate detail!
And yes, kzt is absolutely right. That kind of return is
far too high and far too guaranteed. I think we also need to separate short-term stock trading and long-term investment because they work fundamentally differently.
Suggestion:
Short-term stock investing (actually, any financial instrument) is a lot like poker because short-term stock trading is a zero sum game. Therefore, I'd make it a contested roll somewhat akin to Hacking on the Fly. Use your Knowledge (Financial Instruments) skill as your attribute, then add the rating of your stock trading program. Yes, this means you need to buy or make a stock trading program. There should be special rules for pirating that or making your own, since connecting onto a stock exchange is never free. Now, since you've now connected onto the exchange, you get to name how much money you invest, which can be increased or decreased before each roll. You also name a number X, which will be how much of your money you're aiming to make. Then you make your roll (representing, oh, say, an hour of work), and the GM makes two hidden rolls: one to notice you, then a roll contesting yours with a pool equal to the highest dice pool among those who've noticed you. This cuts both ways: you can make Knowledge (Financial Instruments) + rating of stock trading program to find someone who has made money off of you. If you win
and you get more hits than the size rating of the exchange, you make X% of your principle for each net hit. If you don't get enough hits, you lose X% (or some multiple; I'm making this up on the fly, but it should represent the cost of the broker's transaction fees). If you lose the contested roll, you lose X% for each net hit the other side gets. Like in real life, this means that short-term trading is fairly easy when you first start...but it gradually gets worse when the big players notice you and do their level best to take your shirt. Only, in SR, the big players are
fragging Great Dragons. Note to GMs: stat them accordingly. Lofwyr's pool should involve more dice than you and your players physically possess. Combined.

Note that there are, as usual, various situational modifiers to these rolls. In addition, smaller exchanges should give a bonus to those perception rolls, while very large exchanges might even incur a penalty. Betting big makes you much easier to notice, and so does the simple passage of time. Generally, being noticed is inevitable, so it's as much about knowing when to stop as knowing when to play. You can leverage and short sell, which should have some mechanical effects. Remember the In Debt quality?

Now, long term is a different story, because it is no longer a zero-sum game. However, you have far less control. I'm thinking this is more of an extended roll, but not one based upon the player's dice pools. Let's say...the player picks an amount to bet (call it P for Principle), a type of stock (say, AAA, AA, A, B, etc.), and then rolls Intuition or Logic (player's choice) + Knowledge (Financial Instruments) + any relevant bonuses the GM might think appropriate. This roll picks the portfolio the player chooses to invest in. The GM then each month secretly rolls some static pool modified by the number of hits you get, the safety of the stock in question, and the current economic climate. The pool should be, generally speaking, pretty low, and its max hits limited by the safeness of the stock (so no max hits for a tiny company, but a cap for AAA absent strange circumstances like "PCs just blew up their major competitor's factory" or something). However, always apply exploding six's so there's always a chance of the PCs getting _really_ lucky. Each month, P increases by 0.1% per hit, but if the roll fails or glitches, lose half of P. If the roll
critical glitches, lose 90% of P. If you give your money to a broker, you don't lose anything on a glitch or failure, though you still lose 90% on a critical glitch thanks to the broker either going bankrupt or saving themselves by cutting your loose. However, you only get 0.01% per net hit the broker gets.
Anyways, that's a quick sketch off the top of my head, so the numbers can be fiddled with.