Put Option t withdraw and BSOP model

Hi Sir, I have a question about Pa when we are calculating Put option value using BSOP Model. In my notes from BPP, there is an example where even the cost of a project is being taken into account as Pa. I don't understand that. I am writing down the example for you: Company X is considering an investment in a joint venture to develop high quality office blocks to be let out to blue chip corporate clients. This project has a thirty year life, and is expected to cost Company X £90m and to generate and NPV of £10m. The project manager has argued that this understates the true value of the project because the NPV of £10m ignores the option sell Company X's share in the project back to its partner for £40m at any time during the first 10 years of the project. The standard deviation is 45%p.a., and the risk free rate is 5%p.a. Evaluate the value of option to withdraw. Here the cost of £90m has been added into £10m to get the pv of cash inflows. I don't get how £90m is an inflow. Thanks.