Complex subqueries wihtin CASE statement - mysql

Question

I have this query that calculates the expected value of a receipt, based on the 'channel' of acquisition of the user, on the 'id_store' and on the 'country' of the user.
The resulting query has this columns.
Count | id_store | country | price_usd | channel | T2S | ExpectedRevenue | ExpectedRevenue_projections
(note: ExpectedRevenue = count * price_usd * T2S)
In this query I want to know the value of those receipts that where purchased on August and expire on September (these products have a trial period, and we calculate their value is based on the historical conversion rate (Trial 2 subscription a.k.a. T2S)).
The problem arises when we do not have historical data for some scenarios; either we started selling different with id_stores, new country, channel, or any combination of the previous ones and the T2S becomes as a 'null' value and column 'ExpectedRevenue' therefore has 'null' values' ---> That is why I have a 'CASE' function that makes T2S to take the value of '.28' whenever this happens and I only have to sum the column 'ExpectedRevenue_projections' in order to get the final result.
However, this '.28' is just a fixed value that is not accurate at all (we have products from 10 to 120$...).
That's why I'm asking dear stackoverflow contributors is to replace this '28%' for another T2S that in this case would be calculated historically according to 'country' and 'price_usd' intervals such as (0-15), (15.01-29.99), (30 - 49.99)... and so on.
(The reason to put intervals is that 'price_usd' is calculated based on the exchange rate at the moment of the creation so the same product purchase by a customer in foreign currency could be 49.99, 49.98 , 49.97... and T2S would come as 'null' if there is not historical data).
Thank you
SELECT
r.count AS count,
r.id_store AS id_store,
r.country AS country,
r.price_usd AS price_usd,
r.channel AS channel,
a.count / (a.count + u.count) AS T2S,
r.count * r.price_usd * (a.count / (a.count + u.count)) AS ExpectedRevenue,
CASE WHEN (a.count / (a.count + u.count)) is null then (r.count * r.price_usd * .28) else r.count * r.price_usd * (a.count / (a.count + u.count)) END AS ExpectedRevenue_projections
FROM
(
select count(distinct(r.user_id)) AS count, p.id_store, u.country, r.price_usd, channel
from receipts as r
inner join products as p on p.id=r.product_id
inner join customers as c on r.customer_id=c.id
inner join users as u on u.id=r.user_id
where
MONTH(DATE_ADD(DATE_ADD(r.original_subscription_at, INTERVAL p.trial_days DAY), INTERVAL p.trial_months MONTH)) = 9
and YEAR(original_subscription_at) = 2017
and MONTH(original_subscription_at) = 8
and YEAR(r.expiration_at) = 2017
and MONTH(r.expiration_at) = 9
and r.status='trial'
and p.platform not in ('manual')
and c.tester=0
group by 2,3,4,5
) AS r
LEFT JOIN
(
select count(distinct r.user_id) AS count, u.country, p.id_store, channel
from receipts as r
inner join products as p on p.id=r.product_id
inner join users as u on u.id=r.user_id
where year(r.created_at)=2017
AND r.status = 'active'
group by 2,3,4
) AS a
ON r.country = a.country AND r.id_store = a.id_store AND r.channel = a.channel
LEFT JOIN
(
select count(distinct r.user_id) AS count, u.country, p.id_store, channel
from receipts as r
inner join products as p on p.id=r.product_id
inner join users as u on u.id=r.user_id
where year(r.created_at)=2017
AND r.status = 'unpaid'
group by 2,3,4
) AS u
ON r.country = u.country AND r.id_store = u.id_store AND r.channel = u.channel