How many vertices can your run of the mill $100 gfx card push these days?
What exactly would vertices per second measure? You leave out too many variables.
What would you use the metric for? As a sane default or upper bound for your application's draw calls? For benchmarking? If so, you probably want to benchmark fill rate and triangle draw rate instead. See my post regarding this here: https://stackoverflow.com/questions/1493581/how-to-go-about-benchmarking-a-software-rasterizer/1504635#1504635
On the other hand, if I take your question literally, I sadly don't know of any trustable sources for low-end graphics hardware performance. You surely don't want to trust the vendors themselves. I know of a good usage survey though, and that's the monthly Steam hardware survey: http://store.steampowered.com/hwsurvey/
Since I'm not aware of any sites with good and deterministic benchmarks statistics, knowing which cards (and technology) is popular is the second best thing, if you need some statistics to help you make correct choices for your target group.
Further to Sebastian's comment bear in mind that due to shader hardware the number of vertex transformations is largely irrelevant. What constitutes a transform? You can bet they just do a simple matrix multiplication to a position only vertex (I know Microsoft and Sony did that for their X-Box and PS2 vertices-per-second marketing bumph).
In this day and age the most meaningful things are memory bandwidth and shader cycles per clock. Of course driver are free to re-order instructions to improve throughput. Lots of latencies can be hidden this way which makes the hardware produce faster results.