I’m developing a plug-in and from what I see some hosts offer trial versions and some not.
Does it expected from one developing a plug-in to actually buy all the different hosts out there that don’t offer trial versions?? that’s crazy!
Does it common for a developer to contact those companies asking for a trial vesion?
Paying just Steinberg + Avid + Apple = bankrupcy.
I really don’t get it, cause it’s their interest that I will test the plugin on their host thoroughly…
Offer a time-limited beta version of your plugin to users for free, in exchange for bug reports posted to your forum or bug tracking database.
You can also write to the host developers asking about developer/NFR version for your testing purposes. In many cases - especially when you can make some NFR exchange with your products - they are very keen to give you free versions for compatibility testing. In the case of little host vendors this is much simpler, with the bigger ones the problem is with contacting with the ‘right’ person 
These companies you’ve listed are big enough to have some procedures for this.
Steinberg - no dev program. You can only find the right person.
Avid - there are programs for developers (free) and you can get the SDK and ProTools (assuming you don’t need any Avid DSP accelerated plug-ins - then you will have to pay some fee for hardware)
Apple - basic testing with AUVal is free. Garage Band is preinstalled for every Mac AFAIK. For Logic you can touch someone in Apple.
Anyway you should try
