A proposal sets out recommended work, why it is recommended, and what it costs.
How it differs from a quote
A quote is a price for defined work. A proposal explains the problem, the options and the reasoning first.
That difference is what allows it to be judged on something other than the number, which matters most on larger work where the customer is not qualified to compare the technical detail themselves.
What it contains
The problem, in the customer's own terms, so they can see it was understood.
What is recommended and why, including what was considered and set aside.
The scope: what is included, excluded and assumed.
The price, and what happens if something is found once work starts.
Timing, and what is required from the customer.
Options rather than a single figure
Presenting a good, better and best version generally raises the average value accepted, because it changes the decision from whether to proceed to which option to take — see increasing average job value.
Three is the usual number. More than that defers the decision rather than helping it.
Length
Long enough to answer the questions the customer will otherwise have to ask. Beyond that, length reduces the chance of it being read.
Delivery
Presented and talked through where the work is significant, sent where it is routine. A substantial proposal emailed without discussion is compared on price, which is the one comparison it was written to avoid — see competitive positioning.
Following it
An unanswered proposal is not a refusal — see sales process.