Cost is typically left without the s if there is only one purchase:
The cost of thread
Using costs here would imply more than one type of cost:
The costs of homeschooling
You can still say this without the s but the connotation shifts back to one solitary cost:
The cost of homeschooling
It is ambiguous whether these costs are monetary or emotional or something else. Referring to bundles of purchases will typically use costs:
Needle costs are rising
These are my needle costs
If you used cost here it would shift back into a description of needle markets or a single project's purchase:
Needle cost is rising
This my needle cost
The differences between cost and costs in these examples are subtle and could be entirely regional. One final note is referring to how much something costs. This will follow standard pluralization rules:
The book costs $4
The books cost $4
To directly answer your question, I would use costs: Sewing Material Costs. Variations:
Sewing Material Costs — the money spent on sewing material over multiple purchases
Sewing Materials Cost — the amount required to buy "sewing materials"
Sewing Materials Costs — the amount spend on sewing materials over multiple purchases
Savings is most typically plural in the sense that you are using it:
Our savings on needles are significant
These are my coupon savings
I have $300 in savings
What you want to do is replace "coupon" in the second example with "Material Costs":
Material Costs Savings — the savings on "material costs"
The use of "costs savings" is probably what makes this seem strange. The idea that you are accumulating savings through costs is counter-intuitive. But you aren't saving through costs, you are saving on costs. The alternatives are not quite what you are looking for:
Material Cost Savings — the savings on the price of "material"
Materials Cost Savings — the savings on the price of "materials"
These don't make much sense to me. I don't think "savings" works well with "cost".
Materials Costs Savings — the savings on "materials costs"
This works but the extra plural is even more distracting. This form would work better with other objects, however:
Needle Costs Savings
Car Costs Savings
You can to try to use a hyphen to reduce the awkwardness:
Material-Costs Savings
But I would just stick with Material Costs Savings.
A student can have his or her own individual heavy workload, which is countable, thus they have heavy workloads (plural), or students in general can have a heavy workload (generic, abstract, non-countable) - they they have a heavy workload. Which you choose depends on whether you are considering the students as individuals or generically.
Best Answer
I've never heard anyone use an ending 's' when describing Chinese yuan, Japanese yen, or Italian lira (of course the plural for lira is lire, which sounds about the same.) I've also not heard anyone say bahts. There are other words in the English language where the singular and plural use the same words, such as moose, sheep, and deer, so there's really no reason to create a new plural word if the singular form is already acting as the plural.