Ex post evaluation of conservation projects - accounting for time in cost calculations
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.