There is no dearth of pay as you go SIM deals in the United States. Every network and every reseller of all the major networks have pay as you go SIMs. There are standard per minute and per text charges you would be paying. For internet, some providers charge you per megabyte and some will charge you per kilobyte. Charging per gigabyte is not heard of in the United States, at least in pay as you go deals.
There is one certainty you can rely on while looking for pay as you go SIM deals. None will offer you unlimited internet. There are very few plans in contracts that actually provide you unlimited internet on 3G and 4G speeds. Most will cap the speed to 2G network after a certain point. To the contrary, with pay as you go SIM deals unlimited internet is a given as long as you keep paying and have enough credit. Whatever is the cost per megabyte or per kilobyte of data on a chosen network, which could be 3G/4G or 2G, you will continue enjoying the data till you exhaust your recharge or validity.
What you can choose is the type of SIM, if you would only want data or data along with minutes and texts. For instance, you can go for the H2O Bolt 4G/LTE SIM Card. It will cost you $9.95. You can get 2 GB at $25, 5 GB at $50, 7 GB at $70 or 10 GB at $90. You will be on 3G and 4G/LTE network with this SIM. You can also choose a standard SIM, micro SIM or nano SIM. All the sizes of the SIM cards from H2O cost the same. You may also go for AT&T to get a SIM that only offers data. The SIM will cost $24.95 and you can go for 2 GB at $25, 5 GB at $50 and 8 GB at $75.
If you want pay as you go SIM deals that will have monthly plans then you can choose from various kinds of plans. Some of them will have unlimited data but capped after a certain usage. You will thereon get 2G speeds instead of 3G and 4G LTE speeds. You can check plans of Simple Mobile, T-Mobile, NET10 Wireless and Ultra Mobile. H2O and AT&T also have unlimited data along with minutes and texts in addition to the internet only SIMs.