Being a greenhouse farmer , let me throw in my 2cents.
Having observed the market keenly, I have noticed that a lot of people plant tomatoes in greenhouses more than any other vegetable. I too made the same mistake and started with tomatoes. However the cost of production is high and when you go with your tomatoes to the market, its mostly flooded. I have been doing some research and got to know that things like coriander (dania) , okra, garlic, capsicum, green pepper (pilipili hoho) ,Red Bombay Onion, Rosemary, french beans,can do very well in greenhouses and fetch very good prices .I am using Amiran Farmers Kit Greenhouses as a model here. The crops mentioned above are mostly unexploited by farmers. Saying that greenhouse farming is profitable or not is relative. As we know marketing is not an art many have perfected. I have personally gone to many high end hotels and restaurants and asked them what they want me to plant for them and they told me , and thats what I plant. i have had the two greenhouses in Loitokitok (230km from Nairobi) and the returns so far have been good. If handled correctly , you will be smiling all the way to the bank.
If you think you can, and if you think you can't , either way, you are right!