Question: Cost analysis problem - stuck - flourescent vs. incandescant light bulbs

Guys, I'm stuck on a simple silly problem.  I suppose I have one of those senior moments but in any case  ...

All I was trying to do was derive an equation from a simple word problem I made up.  Specifically it has to do with the fact that it's costing me more to buy efficient energy saving flourescent bulbs than to use those ever so energy inefficient incandescent bulbs.  But it turns out those inefficient bulbs is the cheaper route.

Basically it's a cost analysis problem.  Anyways here's how it goes. 

My 13 watt flourescent light bulbs are costing me $3.67 but only last for 3 months (yes you read that correctly, mine burn out and need replacing in 3 months and I barely even use them ... made in China ..  but anyways) A 40 watt incandescent light bulb costs $3.29 for a two pack.

So all I wanted to do was write an equation that described the cost over time for each bulb and just plot the results.  

Basically   Total Cost = Product Cost + Usage Cost

My most recent hydro bill averaged out to 15.1 cents per kilowatt hour, after factoring in delivery charges, which is rediculously insane by the way.  And I use the light, roughly, let's say 4 hours per day.  Now that incandescent bulb will last a whole year plus more but lets say 1 year, but I need to replace the flourescent basically on time every 3 months.

The usage cost is quite simple (wattage)*(hours of use)*(cost per kWhr)/1000 no problems there.  

But how do I write an equation for the product cost of the florescent bulb?