Reading marginal (shadow) prices
A marginal (shadow) price is the extra revenue the optimiser would earn from one more unit of a scarce resource — an MWh of stored water, +1 MW of turbine, +1 MWh of storage, or +1 MW of reserve-cap headroom — holding everything else fixed.
A value near zero means that limit isn't binding: loosening it wouldn't help, so don't invest there. A large value flags the binding bottleneck — where a relaxed limit or an upgrade would pay back, and roughly how much it is worth per year. They answer: what is holding this plant back, and what is it worth to change it? (Values are for the selected strategy at the as-built size.)