That is, an estimate of the cost of the services, which estimate will not be revised, adjusted, or changed in any way at a later time.

I believe this what it means. Hope it helps.

I see it all the time in advertising from trades people.
Hope is useful.

Commonly found in ads offering services such as home remodeling, appliance repair etc.

The right term in Spanish is:

"Estimacio'n presupuestaria" instead of one in the query.

Good luck!

There is an important difference in the interpretations that have been offered for your phrase - one is that a price will be given that will in no way be changed or modified later (potentially dangerous in the wrong context) and the other, which I prefer and which is more common, meaning that a price will be given and one doesn't need to feel obliged to accept the offer. This latter interpretation depends on the context - a house painter might say ' free estimate'. Other more sophisticated possibilities could be a 'non-binding estimate'. Note that 'presupuesto' is often wrongly translated as 'budget' in English - the difference is that 'estimate' is an approximate PRICE, whereas 'budget' is like a schedule of how a company for example, will spend its money, or the maximum it allocates to an area of spending. So you should check the possibilities offered against your context, before selecting the appropriate answer.

also could very well be "free estimate" depending on the context and just what they're offering - all estimates aren't free even if you're not obliged to go with them.......I'd still say, " estimate without obligation."
Hope this helps! Suerte!