Enough that the funds are irrevocably committed and at risk, which escrow conditioned on visa approval achieves without losing the price if the visa is refused. A non-refundable deposit is also acceptable evidence of commitment, but its size should be negotiated with the visa outcome in mind.
Commitment and prudence are not opposites
The E-2 rule requires that the investor have invested or be actively in the process of investing, with capital irrevocably committed and subject to loss. It does not require the buyer to hand the full purchase price to a seller before knowing whether the visa will issue. The Foreign Affairs Manual recognizes escrow arrangements in which funds are released on visa issuance and returned only if the visa is refused as satisfying the commitment requirement, because the buyer cannot walk away for any other reason.
A seller who wants a non-refundable deposit as well is negotiating price and certainty, not immigration law; a deposit of a meaningful percentage, forfeited only if the buyer defaults, strengthens the case while limiting the loss on refusal. Government costs are the consular application fee and, for the couple's Canadian corporate side, none. Professional fees for the purchase, the escrow agreement and the visa package are business-planning costs; budget them separately from the capital, because fees are not investment.