That is a judgement about proportion. An independent read of the economic analysis and budget is a small expense beside the capital at stake, and the sponsor's report was commissioned to support the offering rather than to advise a subscriber. Budget for it alongside legal fees, government filing fees, translations and family processing.
Decide the diligence budget before you fall in love
Set aside an amount for independent review at the outset, and agree what findings would make the investor walk away. Ask advisers for a fixed scope covering the model, the budget and the allocation mechanics, plus a written summary you can keep. Confirm whether later work — responding to evidence requests, or assembling proof at the conditional stage — is included or billed separately.
None of this reduces the capital that remains at risk. Add one further scope question to that fixed engagement: who assembles the evidence at the conditional stage, and at what price. That work falls years after the investment, when the sponsor's interest in being helpful may have diminished and the adviser who prepared the petition may no longer be instructed, and it is the stage on which the outcome actually turns.
Ask for an indication of cost now, ask what happens if the sponsor does not provide the underlying data, and record the answer with the rest of the diligence. Hypothetical example: an investor obtains a written scope covering the petition and a separate written indication for the later stage, and the difference between the two figures — and the conditions attached to the second — becomes a factor in choosing between two otherwise similar projects.