A little bit of research, as I'm the only one who's bothered to come in to the office today. So I had EF and ba.com windows open anyway ... ;-)

OP doesn't give travel dates, but if we assume before end Mar and purchasing during the sale - i.e. by 26 Jan.

BA publish a thru T fare to SJO 566+321.40 = GBP 887.40. This is only about GBP85 more than the same fare to MIA, compared to the cheapest MIA/SJO Y return on americanairlines.co.uk of around GBP217.00.

DFW is not a valid routing for the thru fare, only MIA or NYC.
Requires Sat nite stay and 21day adv purchase. Usual draconian change/canx penalties apply.

There is no thru I fare for a CW comparison; only J which is silly money.

Diff between T and I fares for LON/MIA/LON is GBP1412, so that is the value of your MFU.

hth