Saying no some of the time is very normal and practical and dare I say useful. If something isn't practical or affordable for your family, and it was really meant to be for your child, he will eventually find a way. I know of an unschooler who at age 3 was taken to the circus and told her family from that day forth that her life's ambition was to be in the circus. I know another one who from the depths of a mud puddle announced that she wanted to build a house. It took many years, and there were lots of "no's" along the way, but the first has done a lot of circus work (tissu -- aerial silks -- being her specialty) and now works full-time on a tall ship where her expertise amongst the rigging is legendary. The other is completing a Master's in green architectural design. Practical constraints are natural obstacles that for some children serve to galvanize, rather than deflate, their long-term ambitions. And if not -- well, it just wasn't important enough to them.
The real world is full of natural obstacles, so I think dealing with them (and occasionally working hard, creatively and long to overcome them) is part of unschooling.
I may be a "no" mom from your perspective, or I may be a "ridiculous lengths" mom. When my eldest dd was 13, her violin teacher suddenly retired with only a week's notice, due to her husband's cancer diagnosis. (She was near retirement age and had only recently married. Their families and the cancer treatment facilities were all elsewhere. We understood her reasons.) There was no one in our region who had the experience and playing skills to teach a student at my dd's level. She had not been practicing much, seemed to be drifting with little motivation, and although she had no end of talent and ability, I just couldn't justify driving hours each way to get her to an appropriate teacher. She wanted to continue lessons but I felt it was not reasonable for all of us (her siblings would have had to come along too) to travel for hours and spend hundreds of dollars for lessons she was only doing a middling amount of work for. She was the most advanced student in our region and when I said no to violin lessons there were many people who thought that was a travesty. Though of course they couldn't really envision how we could realistically satisfy her desire without killing ourselves.
Six months later my dd was practicing 3-4 hours a day, burning with motivation, had identified the teacher she wanted to study with and was hatching a plan for getting by with one lesson every 4-6 weeks. She suggested "buying" the right to a lesson with 100 hours of practicing. She had suggestions for ways to satisfy her siblings' needs during the long trips to her new teacher.
I said yes, and for three years we drove 7 hours to get to her lessons a bunch of times a year. Eventually she was old enough to do long overnight bus rides to get there and back. She continued to practice hard and advance quickly and she is now at 17 living in an even bigger city full-time in order to get lessons with an even-more-advanced teacher.
I think in our case the hardship of the initial "no" made her realize how badly she wanted it, and find ways within reason (albeit barely) to make it work. And it did eventually seem reasonable to support her in her dreams. It seems to have turned out well. She's national calibre now and is likely to win a full scholarship to her university violin performance program of choice.
Who's to know? It's a balancing act. Sometimes you say no, sometimes you say yes. I think you need to consider everyone's needs and make choices that are authentic and considered ... and be willing to change them too as the circumstances change.