When you start especially on higher levels, you will need to advertise or NO ONE will come at all to your store. Junk mail, and one goon is what you need to start with.

At the beginning on higher levels upgrade once the potion maker. You'll need this on days with high demand.

Buy low at the beginning because you will probably lose money quickly since most of the customers don't come to you at first... !!!stock up when the prices are low

Don't worry about upgrading.

Borrow as much as you need from the loan shark and don't worry about paying him back. Later you'll see that you have so much more money than you can ever spend.

Higher your prices (check the minimum price) when the demand is high.

Research. Your other potions will worth more. They're about $100-200 per potion depending on what it cures. Most likely the computer will finish research first but once yours finish higher the price on the days it's high demand. Doing so will easily earn you 1,000 a day.

Easily earning 1,000 a day you can spend on few upgrades but don't forget to stock up.

By this time, you'll get good amount of customers. Check weather, adjust prices everyday, more advertising, and last potion research !!!stock up!!! will lead you to gaining about 2-3k each day.

You can now upgrade as you wish. If you do this every level, you will win every level. The thing is not to over use your money until you get really rich. On the last level I got up to over 30,000 while the others went out of business. xP.