Let me tag @shayden06 for you so he is alerted to your question asking for an approximate cost for the project. Hopefully he can get back to you as soon as possible, although it's been a while since he was last on the site.
You might also want to hit the Start a discussion button and share some photos and a little more detail about your project. I'm sure other members would be happy to provide any suggestions that they can for keeping the costs down as well as ensuring a brilliant result. We have members sharing helpful advice and inspiration every day.
Welcome to Workshop,
@calverts - Hi and welcome. If you look at the photos, you’ll see that the supports for the decking are (I think) sitting on some galvanised fixings of some sort. If you’ve read the other posts, you would have seen that some people were concerned about the type of timber used for supporting the deck. It’s quite feasible to achieve the results of the images above, using better timber and suitable fixings.
I would have had a metaphorical heart attack over those quotes you received
I suggest you start your own topic/discussion in the “Outdoor” forum. This will “stand you out from the crowd” and @Jason can then tag a couple of experienced members to suggest some ideas. I can see what you want to do, I’ve got an idea about how to do it, but as it’s not something I’ve actually done myself, it would be better if others came to the party, cheers Deb