Russian tech blog Rezetked has estimated that the Pixel 3a will start between $400 and $500, which may be correct, or it may not be. Either way, the Pixel 3a is expected to cost less than the original $800 Pixel 3 and the $900 Pixel 3 XL.
As for the larger Pixel 3a XL, it may cost $100 more if pricing curves are similar to the Pixel 3 and Pixel 3 XL price tags.