I'm not sure who has cheapish PN controllers besides Siemens. Note that you theoretically would need programming software in addition to these, which very well may cost more than the PLC. It does have a fully functional 21 day trial, which may be helpful in this case.
An S7-1200 (around a few hundred dollars) supports 1 ms update rates, and if the only program is copying data from one to the other, I can't imagine that taking more than 1ms. However, that means 1ms from transducer, 1ms processing, 1ms send to motion controller, which puts you in a 3ms time range, at best case. You could use a cyclic interrupt to call it at a specific interval (fastest is 1ms), or just let the main program cycle as fast as it can.
A slightly more expensive PLC, like a 1510sp or 1511 (probably around 1k) would support faster bus speeds, Profinet IRT, and isochonous processing. Isochronous processing means that you could set it up where the PLC reads its inputs, does its processing (block moves), and then sends its outputs, all synchronized to the bus cycle (as fast as .25ms). The fastest cyclic task here, if you aren't synching to the bus, is .5ms.
However, one important question is what is the fastest update rate your devices support? Do they support Profinet IRT? Do they support isochronous processing? What profinet class are they certified as?
Long term, the best method would be for the motion controller to be a Profinet controller directly, and supporting I-Device to simultaneously act as a device to other controllers. That doesn't help you right now, though.