I guess I don't see the difference in weight or the hauling. If you are bringing the bags in as pallets as mix or as poured blocks, the weight is the same. I get the travel distance difference, but that would more than be offset by the labor in mixing and pouring blocks and likely the poured blocks are far far cheaper than buying the mix- as you said, money is a issue and the blocks will be cheaper than the mix.
Now I could see a advantage if you don't have big enough equipment to set pre poured blocks, but if you are going to set the forms up on site and pour, you still will have to move and set them, unless you plan to pour in place.
I'm thinking a 3 point mounted forklift mast on the tractor I saw on your trailer, and make the trips to buy the blocks whole, rather than as pallets of mix. If you can't make it up your hill, unload 1/2 of them down at the bottom and make a couple extra trips.
How many blocks do you want to pour? If its just a few you would be better to just make up some forms like the old simons wood forms. The steel forms are for making the same blocks over and over and over again. If you just need to make 10, I'd form up blocks with wood forms and pour as you go. You don't need to buy the fancy forms to make the shape you want. Wood is cheap.
I'm going to suggest if you are making and pouring on site, to not mess with "blocks". Just form it up and pour as you go. The old timers built dams without water stop or anything- with just wood forms. Form up and pour a 4' x 6' by whatever depth your wall is or whatever you feel like mixing up. Strip forms the next day and build on the next piece.
The blocks don't sit tight with each other very well, and if its the wall of your house, a formed and poured wall will be far- far better. And actually easier to do given your site distance constraints.
I was also going to suggest the buying in bulk the sand rock and portland, but then dump trailer will be in order

.