Hey David,
Jumping in kind of late in the discussion. You’ve already received a lot of great advice, so assuming you’re open to a little more – here are my two cents.
It is awesome that you have some demand for your work and expertise in architectural graphics. I wouldn’t worry about charging the right amount at this point. The ‘right’ amount really comes down to how much someone will actually pay. So, I would just be upfront and ask them. Crazy, right?
Focus on developing your personal brand (ie. your name : expertise) and use the opportunity to gain experience teaching your craft. Then leverage it to get your name out there even more, so it can lead you to your next gig. The money will come later.
Also, never be afraid to share your expertise – even for free. Sharing your work and knowledge is the best marketing around. Hence, the purpose of Land8 🙂
Hope this helps!