I took the opportunity to answer this with a video. Hope this helps!
Here's an export of the rule: http://pastebin.com/MyF6vqCa
And the module I mentioned that would make it possible to do this for only select kinds of products is here: https://drupal.org/project/commerce_custom_product
STEP BY STEP
1) Create a price field on the line item type: admin/commerce/config/line-items/product/fields
2) Create a pricing rule: admin/commerce/config/product-pricing/rules/add
3) Add a condition "Entity has field" and select the line item as the entity and the field from the field you created in step 1.
4) Add an action "Set a variable" and choose "decimal" as the variable type, select the source to come from "commerce-line-item:field-YOUR-FIELD:amount-decimal"
5) Add an action "Calculate a value" and choose "added variable" (or whatever you called the variable from step 4 and choose to "multiply" by "100"
6) Add an action "Set the unit price to a specific amount" and choose the "result" variable from step 5