What might be a good estimate for the cost of this landscaping (photos)?

If you took the time and effort into putting this together, you should also have the ability to do your own quote. No one can give you an accurate quote without being able to walk that property and take everything into consideration. sometimes the simplest things can be missed in a picture when it comes to prep work that could end up costing time and money. Also, designing for the landscape should not be a whole lot different than whatever design course you are taking. Basic elements of design generally apply to everything. Balance, color, rhythm, line, unity, flow, etc....words you should be familiar with. That's why your parents have faith in you to design it. you have far too many elements in this for someone to give even a rough quote. Products and materials can vary in different regions. I would suggest trying to call a few landscapers in your area that are willing to walk the yard for free and roughly quote it out for you. Listen and learn or have a recording device in your pocket. I know this sounds sneaky but being resourceful sounds like it may be your only option. good luck to you.