Hi There
I'm the Operations Manager for a Timber Import and Processing company in Auckland, and Excel is my right-hand when it comes to analyzing data or performing tricky calculations. I have developed a number of applications, including one called "Length Spread Calculator" which allows me to examine all of the different lengths of timber I have in a specific species and size, which I choose from a dropdown that lists all of the different types we have in stock. It does a number of other nifty things, but I mention this because it's very similar to your requirements.
What I would do for you is create a "Main Menu" page, where you can select the calculator, which will open up a custom dialog with the dropdowns as you've specified. As you select something from a dropdown, the program will update the other dropdowns with the relevant data from the relevant sheet.
At the bottom of the dialog will be two edit boxes where you enter (1) the number of flowers and (2) the exchange rate (although this could be automated too if you'd like). Et voila! Your calculation is complete.
Other possible features could be a data entry sheet where you can add or update details of the different farms, and draw reports on the current stocks.
If you're interested, please feel free to IM me through Freelancer, I keep an eye on my messages throughout the day, and I am available for a in-depth chat after work (around 4.30 p.m. NZ time).
Regards
Johnvz