“I’m a big fan of structured immersion programs for foreign language.”
I am also.
“They tend to be expensive”
As one example, Middlebury College has a 7 week immersion program that costs $9,995. To me this counts as “expensive”. I did not see however whether they have anything for high school students. What I found was for college age students. The University of Moncton has a 3 week program for high school students (ages 13 to 15) that costs $2,625 in Canadian dollars. I am pretty sure that both include room and board in this price. Travel would be extra for either program. There are of course many other programs.
I don’t think that either of these would be sufficient as a substitute for a year of French, even with some self-study over the summer. I see these as a way to improve your ability to speak and listen to the language, when combined with study in normal French classes taken at the normal progression.