Implementation costs (I.E. the cost for the plugin) is a whole different ballgame to the cost per month or transaction that SagePay will charge your client once it's up and running.
Unless there is a very good reason they want to use SagePay, I'd suggest looking into some other options. It all comes down to volume of sales, and average monthly turnover as to what is best for your client, but I'd look at Stripe as a first port of call. It's a very easy to implement payment gateway and plays really nicely with WooCommerce out of the box. Also, the costs are minimal and all related to actual sales, so she will only ever pay a small percent on the transaction, rather than a fixed monthly cost.
SagePay is easy enough to set up, don't get me wrong, but in my opinion, unless you have a very valid reason to use it, it's overpriced for your client.