Question 1015712
100 cost $225
175 cost $375
200 cost $425

First, since cost for 200 is not twice the cost for 100, there's more going on. 

The 75 from 100-175 are $150 extra or $2 each
The 25 from 175-200 are $50 extra or $2 each

From this we see that we have a linear equation but not "Direct Variation".  DV is when Y=MX, with an intercept at 0. In this case we need to get the Y intercept, which effectively describes a setup fee. The cost before we actually buy the first card. 

We already know cards cost $2 each. Since 100 cost $225, it's a $25 fee

So the equation is Cost = $25 + $2per card. 

250 cards will cost $525.