please correct me if I made a mistake.

please correct me if I made a mistake in writing the sentences below:
1)“Evaluating the finished costs in two years and/or prediction for next year’s is possible with an error percentage”

2)The determinant factor is Selling price of the product on the market.

Without further context, I would guess at possibly:

Evaluating the final costs in two years’ time or predicting next year’s costs are possible providing an error percentage is acceptable.

The determining factor is the product’s selling price when it goes to market.

[color=red]Many many thanks

Hi, teacher: I’m a little confused. Is the main verb are OR is in the sentence above?

 Thanks..