I would probably take these things into consideration.
1 Cost of wood. Probably in your case it would be figured by size required.
A 6" x 18" piece would obviously be cheaper than a 24" x 48".
2 Number of letters and/or numbers required to be cut.
I learned on my "scroll saw cut" key chains that names like Jim or Bill can be sold for about $3.00 but, when I got an order for FIRST AND LAST NAMES with as many as 23 letters it took a considerable more amount of time and effort so I started charging by the number of letters. ($1.00 per letter in my case)
3 Time spent by the hour.
I'm like you, I enjoy the work, and being retired, I'm not trying to make a living but never the less your time and skills are worth something. (If for no other reason than an extra six pack every now and then.
)
4 Your location.
I have found that the price of things has some bearing on how desirable and wanted they are. If they are too cheap, they must not be worth really caring about but, on the other hand if they are too expensive, people may not want to spend the money to own one.
So, if your area has people willing to spend money to have something different or unusual to show off then, you are golden.
5 Adjusting your prices.
When I was in business, I had a friend who was an upholsterer and started his own business. He was very good at his job and within just a few months he was overwhelmed with work and couldn't keep up. I told him to increase his prices by ten percent a month for a year or until he honestly felt that he was loosing work due to pricing. (He later told me that his father had told him the same thing.) He is still in business to this day and is down to working only ten hour days six days a week and making enough money to take a month of vacation every year.
Far better than a few years ago when he was working sixteen hours a day seven days a week and still struggling to make ends meet and loosing his health too.
Anyway, that is my take on what to charge for doing something you enjoy doing anyway.
Rog