I've been asked by my work if I can make my co-workers anniversary cake. . And if so, how much would I charge.

I'm wondering should I charge only ingredients or should I charge for ingredients plus a little more for time?

You should definitely charge for your time.  

Most of us charge for the number of servings per completed design   Pricing depends on SOOO many things like location, your skill, other locals pricing, cost of EVERYTHING that goes into the cake; gas for shopping & delivery; and the list goes on &on   I suggest you price around $3 per serving 

things to consider:

when you do 1 cake for free, for any reason at all...……… you will be required and expected to do all cakes for every one for free.

if you do 1 cake for ingredient cost...…. you will be required to do all cakes for cost.  Have these people done tons of expensive time consuming things for you?

or you may be punished on the job by your co-workers.  SandraSmiley & kakeladi know what they are talking about.

Thank you all for your advice and help. They have not accepted the cake because they don't have the amount I quoted in the budget.

Yah they thought you would give it to them cheap   Don’t become “The cheap” baker   The average person has no idea what goes into making a decorated cake 

Most people have no idea how much it costs to bake a cake, not to mention the time it takes to decorate.  Honestly, because they work in volume and bulk ingredients, your co-workers could probably buy a completely decorated cake from Walmart for less than the materials would cost you to bake from scratch.  It has never bothered me one bit if someone says they cannot afford my quote.  I just don't make their cake.