Hello nairaland structural Engineers,Professional Builders, Quantity surveyors etc.
Please here is a task for our calculating minds.

I have the hard responsibility of casting the top of the soak away/waste water tank in the picture before the rains start.
My bricklayer has however noticed my desperation and is trying to cash in on that to rip me off.

Please house, how much can possibly do the work?
I'll appreciate a breakdown of materials needed and their likely quantities too.
I'm not sure of the dimensions of the tank but standard measurement for waste tanks can be assumed.

More information:-

Granite stones - N35,000
12mm Iron rods and binding wire - N36,000
Planks and plywood for form work plus carpenter fees - N12,000
Cement 9 bags @ N2,000 each - N18,000
Sharp sand - N10,000
Welder workmanship - N7,000
Bricklayers workmanship/labour -N10,000

Grand total - N128,000

Hajj Mufu Sir, over to you!
Trusting your experience and your position as mod to get answers to my query.