Based on prefeasibility numbers, the staged development will initially cost US$439 million, with a subsequent expansion from 13 million tonnes per annum to 19Mtpa costing another $67 million.
Base started 2019 with $49 million in cash and $48 million in debt, with the latter having been reduced by about $44 million over the preceding six-month period.
Base's Kwale operation in Kenya currently has another three or so big years of production left ahead of it to generate cash for building Toliara.
Kwale cost Base around $300 million to develop and was mainly funded by debt.
Toliara is estimated to have a net present value of $671 million - on a post-tax basis and using a 10% discount rate - and an internal rate of return of 22.4%.
However those numbers don't fully capture the project's long operating life, which is initially put at 33 years.
A full feasibility is due to be completed by the end of the year, with an investment decision slated for early 2020 and first production seen as possible by late 2021.
After rising 7% yesterday, shares in Base were down 3% to A30c in morning trade, capitalising the company at $350 million.
At that price the stock is up about 10% on levels it was at 12 months ago, and about 1000% on the 3c shares were changing hands at in early 2016 when there were grave concerns about Base's debt position.