No, not cheap at all.  

I've seen some attempts to do it more cheaply, with some type of laminated wood (thinner plywood, etc) but I'm not sure it saves much, if anything.  (this is "shiplap look" not actual shiplap....in other words, there is no dado cut to overlap. It's just stacked boards, which is how an awful lot of interior shiplap is done. It's not real shiplap.

I've seen it done with pallet lumber, which would be a pita but can look really cool.  Not sure if I could give up a set of knives on a planer to get it smooth.  

I love using old, used lumber.  The floors in half my house were once an old barn.  Made a lot of mistakes, learned a lot, would do it WAY differently.  But still not sure I'd a-pay the cost of having that planed by a pro. (We did it ourselves.) or b-give up a set of knives for it.  We used a floor sander to take down the unevenness (one of the square ones that will get into the corners.  I would not risk destroying a drum on a rental drum sander.)

ETA:  The floor is not meant to be a jack. Just...using anything except prepped lumber ends up taking time and (sometimes) money of a different sort/direction.

There's not much way around the cost nowadays, even if you have a free source of lumber, unless it's perfect when it gets to you.

ETA2:  Even if you were inclined to use some kind of pressed/particle board (which I would not guess you are, just observing the quality of things you have posted) it takes too many coats of paint/primer to hide the substrate, IMO, and...just...putting that on the walls...yeahhhh I just.....no.)

So...you welded that frame?   Will you say what you made it from (I have interest in metal work.  Don't know if I will ever get there due to time, but very interested in what you did and how you measured/anticipated the strength, etc.)

That is RFC.  

(Really. Freaking.  Cool.)

I grew up riding in the truck with my dad, driving over all kinds of crap that held us just fine, but if I looked at it now I would go, "WTF???? NO WAY!"

Yeah...stuff works just fine and has for generations.

Impressed with your attention to detail.  Would love to see your cable system to arch the span.  

ETA:  I have a stoopit "newbie to metalwork" question.

How long would you anticipate your bridge might last (for its required strength) given the rust inside the tubes, etc.  (I mean, as opposed to like..Ibeams or something...it's not like I know what I'm talking about. I just know my trailer tubing rusts inside, so....I'm interested in what people who actually know stuff, would do about stuff. )  This is a serious question, but no math is expected.

Walked out this morning and said under my breath, "For the Love of God stop RAINING for two days please?"

Wasn't supposed to rain last night.

Did.

Too wet to do much today. (I did mow, but mowed wet grass)

Gonna rain again tomorrow.

Might get to work two days this week.

*heavy sigh*

So yeah...wet May.

Will be glad to see your new barn!!!