I am playing a witherbloom aristocrats deck in Timeless.

1. I sacrificed Daemogoth Titan with Tend the Pests to create 11 pests.
2. I cast Beseech the Mirror sacrificing 1 pest for Bargain. Search for Plumb the Forbidden.
3. I sacrifice 10 pests to Plumb the Forbidden, No copies of the spell were made.

As an additional cost to cast plumb the forbidden I may sacrifice creatures. I'm assuming there may be some conflict with not being able to pay additional casting costs, possibly if they are a "may". I see no ruling about it on gatherer for beseech the mirror and possibly I just misunderstand Bargain but why am I given the opportunity to sacrifice creatures if that's a cost I'm unable to pay due to it being part of the bargain spell?