An increase in prices would depend on the number of domestic companies and business engaged in producing the same product, Markets are not suspended because there are no competing imports, far from it, a business concern in WA can still easily out compete a business in say NY simply because of the differences in State and Local taxes and regulatory burdens and mandates.
The real difference would come in the “who” was making the profits, small businesses or global corporations.
Business have to make a profit. If you have many competing competitors, you have duplicated owners, accountants, sales staff, advertising other back office expenses. You most also might have many under capitalized companies that can not afford expensive moder machinery and process. So, you do’t get lower costs from more competitive companies becasuse either they can not, because of small size and low profits with a market that is materially full filled, or they are inefficient and don’t have the excess profits to fund new equipment.
In your case what usually happens is either price fixing, or consolidation, then you might have two, price fixing, or even one company. This is the usual effect seen in many Latin and South American countries.
Also what you basically have in that case is our one company reciprocally cut off from the entire world. Usually those companies have less incentive to innovate.
Your example might be true....for a while. All actions have dynamic reactions/consequences after a while. This is something lefties, RINOs, don’t foresee.
You can drive from northern New Jersey, up to Maine, then accross that width as far south as Pensilvania, and along the lakes to as far West as Detroit/Gary and there are hundreds of dead cities all run on Democrat economic notions.