Heatmap of sold homes with constant cell size?

You are using the Heatmap style renderer, not the plugin. If you want your heatmap to remain the same size, you will need to use the plugin to create a raster file. The heatmap style seems to be affected by the scale of the map canvas which probably explains why the size alters.

This is a good tutorial to follow in creating heatmaps:

http://www.qgistutorials.com/en/docs/creating_heatmaps.html

Heatmaps usually focus on the concentration of points and/or attribute values of those points. If you're only interested in houses with the highest sale price then you could consider interpolating the points using Inverse Distance Weighting (IDW) which you can achieve using the Interpolation plugin:

Then with the output, apply your colour ramp to the style. The following post may also be of interest:

How to build effective heat-maps?