I live in Canada and visit the US seasonally, during which time I typically make several border crossings per month.
In case this interests anyone else, for the last couple years I've used
Ultra Mobile. They have a $19 prepaid plan that was the cheapest I could find and gave me what I wanted. It includes unlimited voice, text and international calling to a limited number of countries (including Canada), and 100MB of up to 4G data. You'll have to
find a store that sells their SIM and there is typically a $20 activation fee.
So far I've found it works well in the more populated areas I've visited, but looking at their coverage map, it would appear Washington and Oregon are somewhat limited. I believe they use the T-Mobile network.
The reason I went with Ultra was the low monthly package price and sufficient coverage in the areas I usually visit, however there have been times while on an interstate in the middle of nowhere, or on small back roads in rural areas, without coverage (which is kind of when I would want it the most, in case something happened with the car, etc.). I travel in the US seasonally, meaning several trips a month for 4-5 consecutive months. I add credit to the account online from Canada by using callingmart.com (they add a $1 service fee) because the Ultra online commerce system is only set up for US billing addresses. Otherwise, it is possible to set up an account with them to add credit, refill plans, buy more data, etc.
Often 100MB is sufficient, but if I use a lot of Google Maps for GPS directions while driving, it can burn through it relatively quickly. The trick, if I anticipate this ahead of time, is to purchase a $29 plan when I refill, and the additional $10 remains as a credit that can be put towards additional data via a text message once the original allotment has been used ($5 for 500MB). Alternatively, when buying the SIM, specify the $19 plan but ask to add more money to the account, in $5 increments. This would allow you to access more data if the original allotment is used before the first month passes.
The biggest problems I've had with them is use of the SIM with a couple different phones, which each needed slightly different APN set-up. The info is on their site, however, if you've just put the SIM in your phone and it's the only way you potentially have to go online, yet you can't, it can be difficult to do so unless you can find a wifi hotspot, etc. For this reason, I would suggest visiting their site before travel to the US and saving the APN info for future reference, or waiting in or near the store where you bought the phone until you can confirm that data is working.
When my next travel to the US will be more than a few months, I'll let the plan lapse, then buy another SIM the next time I am in the US.
BTW, in Canada, I ditched my monthly Telus plan and ported my number over to the
Speakout 7-Eleven prepaid service, which I believe is on the Rogers network. They have some prepaid voice/data plans, but you can also go purely pay as you go for voice at 25¢/minute for local calls and buy a monthly $10 100MB data plan, which if you exceed then costs 10¢/MB. Not the cheapest data out there, but will depend a lot on your typical usage. I'm not constantly checking FB, etc., so it usually works for me. Because I hardly use my phone for voice, it has allowed me to considerably reduce my annual cellphone costs. Another great point about this plan, unlike most in the US and Canada, is that your account credit expires one year after purchase, rather than monthly, and it is possible to activate/deactivate plans when as needed, to preserve remaining credit if you anticipate periods of inactivity, such as extended travel outside of Canada, or if you visit Canada annually, but on an infrequent basis.
