Certainly, the economies of scale of mass production will play a large part in the price differentials of motorcycles and ebikes. However, I suspect that dealer margins and distributor profit form a much larger percentage of the final retail cost. I guess this is driven by anticipated cash turnover and the number of items likely to be sold. Wages, cost of premises, business rates and general overheads are likely to represent a far greater proportion of gross sales value.
I know that quite often, a new £10,000 motorcycle may be sold with a retail profit of only a few hundred pounds or less. Any shortfall will be made up from volume of sales and bonus payments made to the retailer by the manufacturer based on sales numbers against target levels. There is also the likely ongoing income from clothing and accessories, servicing and parts sales, which in the case of ebikes is unlikely to be a reliable income stream.
As well as the economies of scale of mass production, there are economies of distribution. It would likely be much cheaper to import an ebike if you could fill a whole ship with them, and be confident they will all be sold in short order.
Lastly, there is the "Market Price" factor. Most products will be sold at the maximum price that the manufacturer believes the market will pay, regardless of cost of manufacture. After all, why sell an ebike for "£x" if the market is happy to pay "£y"?