To not seem like much on its own is not really about cost per se. Please see below.
It does not seem like much to ask Johnny to clean up his room once a week.
Asking her to drive you to university every week does not seem like much. [implied: to ask]
Items like paper and pens do not seem like much. [do not seem very important in some context. For example: do not seem like too much to order in an office or too much to put in a bag of free school supplies. Too much to ask for from a parent, etc.]
[Some amount of money] doesn't seem like much when you consider quality. [implied: to pay, does not seem like much to pay]
The dress cost $100.00 which doesn't seem like much if you consider why she is buying it. [like much to pay: implied]
50 pens and 10 notebooks do not seem like much. [that number of items do not too large for some purpose]
A sentence can say that a cost "can not seem like much", an amount "can not seem like much", "a quantity** can not seem like much". There it would be about cost, amount or a quantity.
However, if the context is not clear, then, "not seem like much" does not refer to cost, but to some "idea or condition that comes before the utterance in the specific context".
The OP's questions in order are:
1) Items such as paper and pens do not seem (like) much. (grammatical but does not refer to cost. It sounds like: do not seem like much [to order, to put in a knapsack, to ask someone for, etc. etc. etc.])
2) This pen seems like too much. (If someone gives you a gold pen, you might say that. It does not refer to cost. This pens seems like much too (of a gift). Too much to give, too much to reward someone with, etc.)
3) This pen seems very much. [not grammatical at all]
4) This pen does not seem much. [ditto]
This pen is not much. This can refer to cost. And is the only one that could refer to cost without further clarification.
To not be (too) much or to be too much is a spoken language alternative for cost too much. But here the verb is be, and not seem.