Sku Grid doesn't support If/Then Statements, but you can define split ranges.  

Go to Settings --> Marketplaces --> eBay --> US --> Calculating eBay's Selling Price --> Switch to Wizard View Button.

Click on Split Ranges.

Enter the end of the first split, for example it sounds like you'd need a range for 0 - 50, then 50.01 - 80, etc. So your first split is $50. However, you shouldn't just enter 50 there as the next range would start with 50 also and cause conflict.  You can enter something like 50.005 Since that does not exist, then the first range ends technically with $50.  The 2nd range will start technically with $50.01.  Then click OK.

Click on split ranges again.  Enter the split amount of 80.005.  Then click OK.

Click on Split Ranges again.  Enter the split amount of 150.005.  Then click OK.  You won't have to split anymore after that.

You will be left with 150.005 - 99999999999

Then click on the first range. Enter the values in the formula wizard that you want to use to create the formula...I'd use the wizard personally to create your formula instead of trying to write it out -- just fill in all values that you want to account for in your formula. Use estimate price to test the formula by entering a value from your range into the estimator.  

Then click on the 2nd range, define your formula by entering values into the formula wizard. Estimate price.

Repeat for each range.  Once you're done, just click Close on the Wizard.  Save the settings.

Next, you'd want to go to My Items --> Batch Options --> Remove All Per Item Formulas -- to remove any override formulas, if any, from your items and set them back to using the default formula.

Then Batch Options --> Force All items to Reprice.  This will cause the items to be checked and repriced with your new split range formula in your default settings.

For your split ranges to apply, you must not override your items.  They'd have to be using the default formula.

However, you can also create split ranges on individual items when overriding, if needed.