The most time consuming and complicated part of this whole planning process are the visas. Every country has different costs and requirements and procedures and it's kind of like putting together a big puzzle. Again, a great resource for me was Visa HQ.com where you can easily find all the information you need and they can even take care of all of them for you, for a hefty fee. In my case, it would have pretty much doubled the cost of the visas themselves. However, I was able to take care of most of them myself, then left the really finicky one for India up to Visa HQ. (The following info only applies to US Citizens of course and currency is USD.)

Australia
This one is easy. A tourist visa is $20 AUD and can be applied for online on the ETA website. With this visa, you can stay for up to 90 days at a time and re-entry is permitted within 365 days without needing a new one. I received one during my last trip in March, but I've renewed my passport since then and since my passport number changed, I had to apply for another one. But the turnaround is pretty quick, though and I had an approval email in my inbox in less than 24 hours.

SIngapore
This one is even easier. No visa required for stays up to 90 days. All I have to do is book a flight!

Sri Lanka
It's $30 for a double entry tourist e-visa up to 30 days, which is easily attainable online. I talked with my contact in the country since I'm going to be there slightly longer and she said I could easily get it extended there but it will cost between $60-$100 USD.

India
This one was a particular pain for me since you have to apply for your visa within one month of arrival and I'll already be somewhere in Southeast Asia by then. I attempted to figure this all out on their website but it was super complicated so I just hired VisaHQ to take care of it. For a single entry tourist visa up to 30 days, the embassy fee was $60 and the service fee nearly doubled that cost at $49 but definitely worth it for the peace of mind.

Indonesia
Also easy. You'll get a 30 day visa on arrival for $25.

Thailand
Same as Indonesia, you can get a 30 day visa on arrival but I'm going to be there longer than that. My best option is to get the 60 day visa either through VisaHQ (for an additional $79) or at a Royal Thai Embassy or Consulate. Luckily, there is a Royal Thai Consulate in Miami, however with very limited hours, so I completed all the paperwork and actually got approved for a six month visa for $80 (plus about $20 for a prepaid Fedex envelope so they could mail me back my passport since I was moving and wasn't able to drive back to Miami to pick it up.) I was only originally planning to stay in the country for six weeks but I had to get a six month visa since the visa starts from the day you get it approved (which was not mentioned anywhere on the Consulate website that I could find.) So I guess I have a Plan B and can change my plans if necessary and fly to Thailand at any time in the next few months - but this probably won't happen.

Cambodia
Easy enough to apply for a visa online for $40, but you have to apply within three months of arrival so I'll have to wait until September. I'm not too worried since I have a three month window.

VIetnam
Also easy enough to acquire online via myvietnamvisa.com, which is actually a third party that will get you a pre-approval letter for $19.99 so that you are guaranteed a 30 day visa on arrival, which costs another $45 at the airport. You also have to provide 2 passport photos and fill out a form.

The easiest thing it to just outsource all this work and mail my passport to VisaHQ but it would have cost me an additional $325 in service fees. I'd rather spend that money on some delicious street food or on temple tours. The basic takeaway here is that you'll have the least hassle by just staying in places for less than 30 days, but I'm being a little bit stubborn. Overall, I feel like you are punished for planning in advance when it comes to Visas and it's a bit of a Catch 22 because they want you to have all your flights and things booked before applying, but there's no point in booking travel somewhere if you don't get approved for your visa. Que sera sera.