Is carpentry worth it?

At the most basic level the profit from making license plates or superior beds is simply retail price - materials cost.

License plates use $10 of sheet metal to make 2 plates which sell for $20

Trees cost $100 and produce 2 superior beds sold for $800

Although the profit margin is greater it requires more investment (both time and money) to get to the position of producing beds, and this is where it becomes more difficult to calculate if it's worth it.

Both plates and beds need a workshop and for prisoners to complete the workshop safety induction, so we can discount those costs.

You also mention a forestry

... requires the attention of non-prisoner workers which need to be paid.

Metal deliveries have to be removed from the trucks by Workmen, so some paid staff are required, but if your delivery area is inside the prison prisoners will transport the raw materials to the workshop during working hours.

For a Forestry you still need workmen to get the saplings off the truck, but you also need Gardeners to plant them. The Workman are needed again to chop down the trees, so the additional staff required are Gardeners plus the wages for all staff involved while they are performing these tasks. Discounting the Workman from the equation each Gardener will cost $500 + $100 per day, and to be efficient you are going have to hire at least 5 (IMO)

I'm not going to add in the cost of an additional workshop building, as you could replace presses with carpentry tables. Saws, Presses and Carpentry tables all cost the same ($1500) although carpentry tables do take up slightly more space. The problem is the game will nag you that the workshop is not "complete" without a workshop press.

To summarise:

Prisoner carpentry training - $?? Can't remember if there is a charge, since you
                               have already hired the Foreman
Carpentry table             - $1500 each
                               (can be offset by not buying workshop press)
Gardeners                   - $500 + £100 per day per gardener
Tree                        - $100 each (takes time to grow)

So (in a simple calculation) the biggest cost is the additional labour required by the Gardener, compared to license plates.

I don't know how long it takes for a tree to grow but for the purposes of showing a calculation let's say it's 3 days:

Profit = 2 Beds   $800
         - Tree   $100
         - Salary $300 
                  ----
                  $400

Where salary = Gardener $100 * 3 days

I'm not including the Workman salary as he's always required if you are producing license plates or beds.

So there's still a good profit to be made from making beds and there are economies of scale. Once you have a large enough forest and enough carpenters to be working on lots of beds then it will make more money than license plates.

To answer you question "is it worth it?", I'd say yes. Not only financially but also from the fun in playing the game and using all the features.

Additionally, in the blog post for Alpha 18 Chris said

In future alphas we’ll be making use of this furniture

Although I'm not sure this will be of much use given how long it takes to get up and running making beds. All my prisoners will be sleeping on the floor for weeks before the first bed is produced :)