I don't think it was just "pros". I had assemblers during my high school years. In fact Atari shipped one as a cartridge

http://en.wikipedia.org/wiki/Atari_Assembler_Editor

With which I wrote several programs during my high school years. The thing that sucked about the Atari Assembler Editor is it worked like basic. Lines were numbered. You'd type LIST to see your program each line prefixed by a number. LIST 100-200 would list lines 100 to 200. Etc.. But you could renumber lines to insert stuff if you ran out of line numbers.

That was replaced by the Atari Macro Assembler.

http://www.mixinc.net/atari/amac.htm

Which I also had in my high school years before going pro. It was pretty similar to modern assemblers.

I don't remember the price but I wasn't rich so it couldn't have been that expensive. It's possible I pirated the Atari Macro Assembler (thought I don't think so) but I couldn't have pirated the Atari Assembler Editor Cartridge :-)

There were also assemblers for the 48K, but they weren't that friendly to use, when compared with other systems.

Many Pros used more powerfull systems and downloaded the code via the extension interface.