Ex post evaluation of conservation projects - accounting for time in cost calculations I'm looking at differences in the cost-effectiveness of two alternative conservation strategies (treatments). The analysis will compare benefit-cost ratios between the two treatments. Projects vary in age (number of years since undertaken eg. trees planted) within and between treatments, from 20 to 7 years. I have several projects as replicates for each of the two treatments. I have field data (birds) for my measure of effectiveness, measured in one year. I have calculated a total cost using input costs from an established pricing structure, giving a total cost for each project in dollars equivalent to the same year as the bird data. I do not have access to the original (actual) cost for the year undertaken. Note the costs were one-off in the year of implementation, not ongoing.
My question relates to the estimation of costs and the different ages of the projects. My concern is that similar projects will have the same cost, despite one being twice the age. Should the costs account for different ages of the projects, and if so how to approach it? The literature commonly focusses on calculating present value using discounting for projects to occur in the future; I can find less regarding past costs.
If for project $P_i$ the cost calculated was based on prices that held during the year that the project was executed, then certainly you should bring all costs on the same temporal base, in order to compare them.

This can be done in different ways, namely, the common base may be the year of the oldest project, or the year of the most current project, or something in between. Choose the one that will make more sense to the recipients of your study: this usually means bringing "past costs to the present", because this facilitates comprehension (people can better understand current costs and to what they amount in term of resources), and also, makes the adjusted figures more relevant for future projects.

In order to bring past costs to the present, you can use a price index like the Consumer Price Index, and the implied inflation. In general, costs in current dollars is just

$$\tilde C_{i,t|t+k} = C_{i,t}\cdot \prod_{j=1}^k(1+\pi_{t+j})$$

where $t$ is the actual year the project was undertaken, and $t+k$ is Now. $\pi_{t+j}$ is the price inflation between year $t+j$ and year $t+j-1$. Data on yearly inflation are usually available and easily accessible.

Another possible issue to consider is whether all these projects were undertaken in the same economy/country. If they are not, then you also face the task of incorporating in your calculations "purchasing-power-parity" considerations.