I met the same problem as you.
I push two tables result into t1 object and group t1.
from p in Products
join bp in BaseProducts on p.BaseProductId equals bp.Id
select new {
p,
bp
} into t1
group t1 by t1.p.SomeId into g
select new ProductPriceMinMax {
SomeId = g.FirstOrDefault().p.SomeId,
CountryCode = g.FirstOrDefault().p.CountryCode,
MinPrice = g.Min(m => m.bp.Price),
MaxPrice = g.Max(m => m.bp.Price),
BaseProductName = g.FirstOrDefault().bp.Name
};