Economics teaches us that a big difference between cost and price attracts competition which should make the price trend towards the cost.

Practice taught me that that "should" is doing a lot of heavy lifting here and it's often not the case, even across long time periods (years) that should allow competitors to emerge.

For example I calculated the cost of a solar install to be approximately: Material + Labour + Generous overhead + Very tidy profit = 10,000€

In practice I keep getting offers for ~14,000€, which will be reduced to 10,000€ with a government subsidy and my request for an itemized invoice is always met with radio silence.

Only if the barrier of entry is low.

Which it won't be, if at every turn you choose the hyperscaler.

If this is the case, cheap bandwidth for AWS, when?

A big difference between cost and price is often won at the expense of many years of concerted R&D, though

Economics has a lot of other lessons teaching us why prices of major clouds have remained somewhat expensive relative to cost

Exactly.