Woods gives you 10 each day, so the expectation value there is pretty simple. Deep Woods isn't much more complicated; that gives 20 two-thirds of the times, for 40/3. Mountains, though, are much more complicated. The expectation value there is 100/3, but that's the gross amount. You're losing an elf, and the elf's replacement value is 75. If we subtract that from 100, we get 25, giving 25/3.
However, the replacement value isn't always relevant. If we use the value of 100/3 = 33.33, that means that an elf takes over two days to make back their cost. So we don't want to buy any elves when there are fewer than three days remaining. On the last day, going by expectation values the best strategy is clearly to go Mountain (it doesn't matter if they die, since they aren't getting any more trees even if they survive). Since on the last day, an elf has an expectation value of 33, we should subtract that from 100 to find the Mountain expectation value on the second to last day. That gives (100-33)/3, or about 22, which is larger than Deep Woods. Thus, at the beginning second to last day, the expected money from each elf for the next two days is 33+22=55. Thus, the expectation value for Mountain on the third to last day is (100-55)/3, or about 14.8, which is still larger than for Deep Forest. For the fourth to day, we have (100-(33+22+15))/3 = 9.87, which is lower than Deep Forest. So we should send the elves to the Deep Forest up until the third to last day, then send them to Mountain for the last three days.
Now, when should we stop buying elves? Well, the expectation value of elves at the beginning of the third day is 70, so we shouldn't buy any elves then. But at the beginning of the fourth to last day, the value is 84, so we should buy them.
All of this is based on expectation value calculations. However, if your goal is maximize the probability of having the highest amount, rather than maximizing your expected value, then the calculation is more complicated. Then it depends on how many other players there are, and what their strategies are. If everyone else are going for the above strategy, then you'll all be tied at the beginning of the fourth to last day. So it would be a good idea to send all your elves to Mountain then: you'll have a one third chance of dropping to last place, and a two thirds chance of jumping ahead of everyone else.