Make it: MAX(sales price) and your second calculation should work.
Can't you even define a 'threshold' parameter? Meaning define an appropriate range for your sales as threshold and show quick filter, so that a user can change the slider on the filter to change the sales value and the graph updates 'true' or 'false' values. In the calculated filed apply the formula:
sum([Sales Price]) > [Threshold]
You sure can. You could use avg([Sales Price]) in your first calc, and replace the hard coded value with your threshold parameter. Note, I'm assuming avg is the best aggregation choice, but depending on what you are going for, something else might work better.
In the second calculation, avg([Sales Price]) may be a token aggregation in a scatterplot (since such views tend to be relatively disaggregated). Something like:
IF avg([sales price]) < Total(avg([sales price]))
THEN "Too Cheap"
ELSE "Price ok"
might do the trick. I'd need to see your setup to comment on partitioning for this calc, but the default may work for you.