When I sold my cakes I spent a long time researching my startup and ongoing expenses, not just ingredients and hourly wage but also initial investments (pans, boards, tips etc) wear on pans, buying new tools as the old ones wear out, or perhaps a specialty design requires a new purchase etc. You need to start there. Done correctly, it should be painful and time consuming! That means you’re being thorough There are a ton of sites online that will help you find the number of servings per pan size, as I assume that is what you meant by selling “by the slice”. If I am mistaken kindly disregard my following suggestions! So, IF you are pricing your orders (entire cakes) by the serving you need a chart(s) and you need to stick with it. I use two, one for party cakes and one for very complicated cakes. Cakecentral is, as others have said, an awesome resource but remember, your local market dictates what you’ll be ultimately working with (or against) and also remember there are a wide array or talents there and everyone has an opinion on what is too little or too much some leaning one way or the other. You ahve to always do what is right for you in the end, and be able to take the suggestions you get and ultimately form your own opinion rather than just following to the “t” what anyone says is “right” to charge.
The next step is a market survey, what the other cake shops charge etc and how their product compares to your work. If you intend to stick around, you might want to avoid making enemies right off the bat by undercutting them especially ESPECIALLY if you are doing this out of your home and they have to charge more due to being legit. I personally don’t see anything wrong with “trying on” a business idea from home at first with friends, family and referrals from them, but if you decide to be a real business and compete with the locals for an extended more permanent amount of time you should consider applying for the applicable licenses/permits. Once again, only my opinion and I don’t know what your plans entail at this point, if that is already something you have done/planned to do but based on you being here asking about this I am going to guess you’re not to that point quite yet. You’ll get there!
Some other things to consider are mark ups/add ons. Consider the price point of a flavor change from vanilla with buttercream to fondant and the extra time it takes, or a buttercream filling to a blueberry filling or a design that requires a 4 layer 10’’ versus one that requires a carved cake which in the end cannot be easily priced by the traditional way when pricing by the serving amount of a particular pan size. What about hand modeling, gum paste/pastillage flowers (long hours and meticulous work if you’re like me and want each one perfect) hand painting, razor work and the like. Figure out how you feel about those tasks and how much extra you would want if they were added to a cake. Only you can decide that, or if you even want to have ad ons or just make inclusive prices. Trust me, cakes get complicated FAST especially when someone knows you’re good! Don’t sell yourself short and get burned out. I did, even though I learned my lesson quickly and corrected and eventually had a great booming cake business, I got so burned out I decided to stop. I was making good money too, but it is hard work, you know that of course. Just don’t sell yourself short.
Ok, enough ranting. I know not everyone will agree with everything I have said here and that is their right, but take it for what it’s worth from someone who had a successful business. This is just how I did it, in very basic rusty terms.