10, 10.1, 10.01, 10.001, 10.

is this arithmetic sequence?

For a sequence to be arithmetic, the common difference has to be the same - you have to add the same number each time. For example, 2, 5, 8, 11 would be an arithmetic sequence because you are adding number 3.
Since this is not the case here, this sequence 10, 10.1, 10.01, 10.001, 10.0001,  is not arithmetic. 

If on Monday it is $3.50 and the price dropped down on Tuesday by $.50 It would be $3.00 if on Wednesday it increases by $.65 it would be $3.65