Gather Aggregate Data From Two Tables

I have three tables:

1) Contract - columns ContractNo and ContractName

2) SalesTransactions - multiple rows for each contract - relevant columns: ContractNo, InvoiceDate, Value

3) CostOfSaleTransactions - multiple rows for each contract - relevant columns: ContractNo, TransactionDate, Cost



How do I write a SELECT statement to produce rows containing:

ContractNo
ContractName
Sum of Value for ContractNo between @FromDate and @ToDate
Sum of Cost for ContractNo between @FromDate and @ToDate

Not all Contracts have either Sales or CostOfSales Transactions in the relevant date range and so one or both totals can be zero.

I've written something like:

SELECT CT.ContractNo, CT.ContractName, sum(CT.Value) as TotalValue, Sum(CS.Cost) as TotalCost

FROM Contract CT

INNER JOIN SalesTransactions ST ON CT.ContractNo=ST.ContractNo

INNER JOIN CostOfSaleTransactions CS ON CT.ContractNo=CS.ContractNo

WHERE (ST.InvoiceDate BETWEEN @FromDate AND @ToDate) AND (CS.TransactionDate BETWEEN @FromDate AND @ToDate)

GROUP BY CT.ContractNo, CT.ContractName

The TotalValue and TotalCost figures I get are much higher than expected. I presume this is something to do with the JOINs or WHERE clause. Please can you advise how I get the correct values?