The average Filipina thirty-something seeking to marry a foreigner are either single moms who have had unsuccessful relationships with Filipino men. Or single women who simply feel the pressure to marry. In the provinces, this would already border on the spinster age, since most girls would often marry young and be housewives. In the city, however 30 is not much of an issue, as the women are indeed more confident and focused on their work.
The "stereotype" reasons why Filipinas are into marrying foreigners would be- a change in their financial status, a chance to secure a foreign visa and live abroad (a common barometer of success in the culture). There's also the likelihood that their biracial children can enter local showbiz or become highly paid ahletes, their offsprings would be instantly branded beautiful if they are half caucasian or taller than most pinoys. Or the Filipina in question might not be considered a beauty among Filipinos, but a foreigner might find her exotic features simply beautiful, this can give her a good chance of finding a spouse who would love her for what she is.
There's always the love factor you can mix in the equation. The love factor would easily come in the heart of a pinay once the idea of a better life and future can be met. I am stating this as a stereotype, not every 30 something pinay may have these reasons for marrying a foreigner.