Consider checking out an ARM Cortex-M board - you can get an STM32F103 'blue pill'[1] with 20KB RAM, 64KB flash, external oscillators, and a 72MHz clock speed for $5. Plus, it has lots of extra peripherals like a realtime clock which can run off of backup power and periodically wake the chip up from sleep.
The future of embedded development is so bright, you gotta walk around with sunglasses on.
If you still want to use Arduino, I'd highly recommend going for Arduino clones from China. You should be aware that the USB chip is different so you may have driver issues, but I haven't had any issues on Linux. You can get an Uno clone for ~$3.
It's also worth considering moving to the Atmega by itself You can get the chip used in the Uno for $2, set it up on a breadboard, program it with an Arduino as the ISP, still use arduino libs if you want, and eventually move to a custom PCB.
They are way more capable, and they actually cost less than ~$3 each for a simple QFP48 STM32F103 or STM32F051 board, and the smaller chips start around $0.75 for my PCBs.
I was mostly thinking about alternatives for people who are currently using Arduino and want to keep the same toolchain, libraries, etc. but at a lower cost. If someone is comfortable moving outside the Atmega ecosystem, I would recommend ARM chips as well.
Arduino port is just wrapper around stdperiph HAL...
Having a small device with WiFi makes it a lot easier to do interesting things - such as pulling data via Web-based APIs, and submitting readings of sensors to MQ, etc.
Having a "real computer", like a PI, or a blue-pill is nice, but suddenly there is a lot more to think about.
Because I can't justify learning the nitty-gritty of some proprietary 'Tensilica' core. Lately I've been meaning to look into using NRF24L01 transceivers for wireless communications in projects, since breakouts for them are less than $1 each and have the same footprint as an ESP-01 module.
I've also heard good things about NXP's Cortex-M chips - apparently they let you map peripherals to whatever pins you want, which sounds neat. And TI's MSP432 chips apparently have fantastically low power consumption for their performance. Atmel's SAM chips look less promising, with minimal peripherals besides a weird common communication bus that seems to handle things like I2C/SPI/etc, if I'm reading the datasheets right.
Does that sound right? It'd be nice to learn about all of them, but there are only so many hours in a day.