What decides what custom software actually costs?

One: how many different people use it

Software for one team is a fraction of the work of software several groups use differently. The moment a customer, a member of staff and an administrator all need their own view of the same thing, you have three interfaces, three sets of permissions and three ways for the wrong person to see the wrong record.

This is usually the single biggest jump in a quote, and it is the one people least expect, because from the outside it still sounds like one system.

Two: whether money moves through it

Taking payment changes the standard everything is held to. Failed payments, refunds, part payments, someone paying twice, someone disputing a charge months later: all of it has to be handled, and none of it can be handled approximately.

It is not the payment button that costs. It is everything that has to be right around it, and the fact that a bug there costs real money rather than an apology.

Three: how much has to survive being wrong

An internal tool where a mistake means somebody retypes a row is cheap. A system where a mistake means an invoice goes out wrong, or a booking is double sold, or a record is lost, is not, because most of the work is in the checks rather than the feature.

Ask any quote what happens when something fails halfway. If nobody has thought about it, the number is lower than the job.

Four: what it has to talk to

Standing alone is cheap. Every existing system it has to read from or write to adds work that is invisible in a demo: the format they expect, what happens when they are down, and keeping two records of the same thing from drifting apart.

Integrations are also where estimates go wrong most often, because the difficulty is not in your control and cannot be fully seen until you are in it.

Five: whether anyone is looking after it afterwards

A build that is handed over and abandoned is cheaper on the day and more expensive within a year. Browsers change, dependencies get security patches, the business changes, and none of that stops because a project finished.

A quote with nothing after launch in it is not a lower price for the same thing. It is a different thing, and the difference lands on you.

How to read a quote you are given

A quote worth trusting says what it does not include. Vagueness on scope is not generosity, it is the room where a fixed price turns into a variable one, and the conversation about that happens when you are already committed.

Be equally wary of a day rate with no ceiling. It is not a price, it is a meter, and the incentive it creates points the wrong way.

The most useful question is simply: what is the thing you are least sure about, and what happens to the price if you are wrong? Anyone who has actually estimated the work can answer it in a sentence.