A dimmer on the hallway light can make the night walk easier on the eyes, because a softer light is less harsh when someone gets up half asleep. The switch itself is rarely what drives the bill. The wall, the wiring, and the number of switches do.
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The wall box is the first question
A dimmer needs room in the existing box. If the box is shallow or crowded with older wires, the electrician may need to replace the box or open more of the wall to make the switch fit. Ask whether the estimate includes the box, and if it does not, ask what a replacement would add to the bill.
Check the wiring at the switch
Many dimmers need a neutral wire in the switch box, and not every older box has one. If the wire is missing, the electrician may need to run a new one, which adds time and access work. Ask the electrician to tell you, before the job starts, whether a neutral is present and what happens if it is not.
Count the switches that control the light
A hallway light is often controlled from two places, such as the bedroom door and the bathroom end, using a pair of switches that work together. A dimmer changes how that pair works, and the wiring can be more involved than a single switch. Ask how many switches control the light and whether the estimate covers each one.
Look at the circuit and the fixture
Some fixtures do not dim well with every switch, and some dimmers do not work with every bulb type. The result can be flicker or a light that will not go low enough for a night walk. Ask the electrician which fixture and bulb type the dimmer is rated for, and write the answer on the estimate so the choice is not left to chance on the day of the install.
Local permits and the electrician's visit
Permit rules for electrical work vary by city and county, and the estimate should say whether a permit is part of the job and who pulls it. A visit fee, a minimum charge for a short job, and travel time can make a small change cost more than expected. Ask for the total on the page, with each of those lines named.
What to compare before you choose
Compare the estimate lines for the box, the neutral, the number of switches, the fixture check, and the permit. The switch model is the easiest line to change and the least likely to move the total. Keep the lowest light level in mind as well, so you do not pick a dimmer that goes too dark to see the floor on the way to the bathroom.