If you use the slider and link it to your price attribute, this should automatically update the price field.
If you want to use the slider eg for an amount and recalculate the to total price, look at the events tab in the slider and create on change behavior to calculate the price.
On change nanoflow (example is using base price as part of the product)
I think you would want to go for a structure like this:
When the customer places an order, they can select several products to order. For each Product they choose, an OrderLine is created. The OrderLine has an association to the Product where the BasePrice is stored (configurable by an administrator probably in a product catalog). For the OrderLine, you can now use the slider for the amount and calculate the LineItemPrice based on $OrderLine/Amount * $OrderLine/Export.OrderLine_Product/Export.Product/BasePrice