The equivalent verb is actually presuponer - to estimate or to quote where one presupposes what it is going to cost.
You may also use estimar for to estimate.
There are two related terms:
underestimate (subestimar (vtr), cálculo muy bajo o cálculo muy menor (grupo nom)),
and overestimate (sobrestimar (vtr), costo sobreestimado (loc nom m)).
Congratulations matthew. You're the only one to spot my mistake in the last six months (including me!). The infinitive of the verb is of course presuponer (as you say) and not as I said; there is no presupuestar.
Presuponer (and suponer that it comes from) is very irregular - but not as "irregular" as I was! Thanks for catching that. I've now corrected it.
When I see the word, "presupuesta", I immediately think of the weird, "presume."