Check the STM32F103C8T6 development board, cost about $ 1.5, and is faster/has more memory than the Arduino Uno, and is much smaller.

It can work with the Arduino IDE, but not all libraries are supported, so check before you want to use it if your used libraries are supported, or if there are different libraries you can use instead.

**Update**

Differences:

                           Arduino Uno       STM32F103C8T6 Mini Dev Board

    MicroController            Atmega328P           Cortex M3
    MIPS/MHz                        1                   1.25
    Operating Voltage [V]           5                   3.3, most pins 5V tolerant
    Input Voltage   [V]            7-12               USB only
    Input Voltage (limit) [V]      6-20                (USB)
    Digital I/O Pins               16                   37
    From them:PWM Digital I/O Pins  6                   12
    Analog Input Pins               6                   10
    DC Current per I/O Pin [mA]    20                    ?
    DC Current for 3.3V Pin [mA]   50                    ?
    Flash [KB}                     32                   64
    SRAM [KB]                       2                   20
    EEPROM [KB]                     1                    -
    Clock Speed [MHz]              16                   72
    
    I2C                             1                    2
    SPI                             1                    2 (18 MBit/s)
    UART                            1                    3
    CAN                             0                    1
    USB                             0          USB 2.0 FS 12 MBit        
    DMA                             0          7 channels (ATsP, SPI, I2C, USART)
    RTC                             0                    1
    CRC                             0                    1
    Unique ID                       0                    1

    Programmed by                  USB                 SWD Debug (around $5)

    IDE                         Arduino IDE         Arduino IDE, CubeMX, ...
    Libraries                      Many               Limited
 
    Dimensions  [mm]            68.6 x 53.4           53 x 22
    Weight       [g]               25                    ?
    Price (cheapest)  [$]         2.50                 1.50

Note: as written earlier, despite the mostly better specs, not all libraries are supported. Also a programmer need to be bought (only once, cost about $5).