Our experience (based on simulated data) with Summed Pricing is that the +/-30% shock will indeed give you enough information in the design matrix to estimate the price slope (sensitivity) independent of the attribute you are attaching level-based prices to. In the BYO question, respondents will see the $100, $150, etc. prices you specified on each level. But, in the rest of the survey, random shocks will occur.
Some researchers have been willing to do less random shock (say +/- 20%) because they are willing to give up some precision in the utilities because they maybe have large enough sample size to still obtain good enough precision in terms of aggregate measures (shares of preference from simulations, for example).
For more on our simulation results, please see https://www.sawtoothsoftware.com/downloadPDF.php?file=price3ways.pdf