Volume and profitability go hand-in-hand on Google Shopping in some respects. The answer to this question is purely mathematical and will be different for everyone.
Selling 10 products with £10 profit per unit at £5 CPA is still going to be as profitable as selling 1 product with a £50 profit margin at £25 CPA. Ideally, we would like to be somewhere in the middle, where we can sell 10 of the product with £10 in profit AND 1 of the product with £50 in profit. It’s important to have a mix of products that are profitable, less valuable, and higher volume, as well as some products which may not receive as much volume, but will be more profitable per unit sold. We also use this mix in our Google Shopping. We have some products that sell for upwards of £1,500, however, these sales are much less dependable than the lower-priced products. Whilst we can depend on selling a few thousand pounds worth of items sold at £200 – £500, the more expensive products are less regular, predictable, and dependable.